diff --git a/keyboard/hhkb_rn42/MEMO.txt b/keyboard/hhkb_rn42/MEMO.txt
index 100b4442d..86f5faef2 100644
--- a/keyboard/hhkb_rn42/MEMO.txt
+++ b/keyboard/hhkb_rn42/MEMO.txt
@@ -8,6 +8,7 @@ Roving RN-42
 
 TODO
 ----
+- LUFA sendchar should be buffered and serial_uart.c buffur size is too large(256).
 - Factroy reset doesn't work; need to test again. 10K pull-up is too high?
 - Lipo voltage ADC sensing
 - Lipo charger configuration: fast charge time:  USB charger spec?
diff --git a/keyboard/hhkb_rn42/config.h b/keyboard/hhkb_rn42/config.h
index cd8fc176d..e641a5a2b 100644
--- a/keyboard/hhkb_rn42/config.h
+++ b/keyboard/hhkb_rn42/config.h
@@ -57,20 +57,23 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
     #define RTSEN 0
     #define CTSEN 1
 
-    #define SERIAL_UART_BAUD       115200
-    #define SERIAL_UART_DATA       UDR1
-    #define SERIAL_UART_UBRR       ((F_CPU/(16.0*SERIAL_UART_BAUD)-1+0.5))
-    #define SERIAL_UART_RXD_VECT   USART1_RX_vect
-    #define SERIAL_UART_TXD_READY  (UCSR1A&(1<<UDRE1))
-    #define SERIAL_UART_INIT()     do { \
+    #define SERIAL_UART_BAUD        115200
+    #define SERIAL_UART_DATA        UDR1
+    #define SERIAL_UART_UBRR        ((F_CPU/(16.0*SERIAL_UART_BAUD)-1+0.5))
+    #define SERIAL_UART_RXD_VECT    USART1_RX_vect
+    #define SERIAL_UART_TXD_READY   (UCSR1A&(1<<UDRE1))
+    #define SERIAL_UART_INIT()      do { \
         UBRR1L = (uint8_t) SERIAL_UART_UBRR;       /* baud rate */ \
         UBRR1H = ((uint16_t)SERIAL_UART_UBRR>>8);  /* baud rate */ \
         UCSR1B |= (1<<RXCIE1) | (1<<RXEN1); /* RX interrupt, RX: enable */ \
         UCSR1B |= (0<<TXCIE1) | (1<<TXEN1); /* TX interrupt, TX: enable */ \
         UCSR1C |= (0<<UPM11) | (0<<UPM10);  /* parity: none(00), even(01), odd(11) */ \
-        UCSR1D |= (0<<RTSEN) | (0<<CTSEN);  /* RTS, CTS */ \
+        UCSR1D |= (0<<RTSEN) | (0<<CTSEN);  /* RTS, CTS(no flow control by hardware) */ \
+        DDRD |= (1<<5); PORTD &= ~(1<<5);   /* RTS for flow control by firmware */ \
         sei(); \
     } while(0)
+    #define SERIAL_UART_RTS_LO()    do { PORTD &= ~(1<<5); } while (0)
+    #define SERIAL_UART_RTS_HI()    do { PORTD |=  (1<<5); } while (0)
 #else
     #error "USART configuration is needed."
 #endif
diff --git a/keyboard/hhkb_rn42/rn42/rn42_task.c b/keyboard/hhkb_rn42/rn42/rn42_task.c
index 7ec4c1b54..2bc1c7fd0 100644
--- a/keyboard/hhkb_rn42/rn42/rn42_task.c
+++ b/keyboard/hhkb_rn42/rn42/rn42_task.c
@@ -56,9 +56,7 @@ void rn42_task(void)
         // Config mode: print output from RN-42
         while ((c = serial_recv2()) != -1) {
             // without flow control it'll fail to receive data when flooded
-            rn42_cts_hi();
             xprintf("%c", c);
-            rn42_cts_lo();
         }
     } else {
         // Raw mode: interpret output report of LED state
diff --git a/protocol/serial_uart.c b/protocol/serial_uart.c
index 600340cd8..35df27fd6 100644
--- a/protocol/serial_uart.c
+++ b/protocol/serial_uart.c
@@ -41,6 +41,22 @@ POSSIBILITY OF SUCH DAMAGE.
 #include "serial.h"
 
 
+#if defined(SERIAL_UART_RTS_LO) && defined(SERIAL_UART_RTS_HI)
+    // Buffer state
+    //   Empty:           RBUF_SPACE == RBUF_SIZE(head==tail)
+    //   Last 1 space:    RBUF_SPACE == 2
+    //   Full:            RBUF_SPACE == 1(last cell of rbuf be never used.)
+    #define RBUF_SPACE()   (rbuf_head < rbuf_tail ?  (rbuf_tail - rbuf_head) : (RBUF_SIZE - rbuf_head + rbuf_tail))
+    // allow to send
+    #define rbuf_check_rts_lo() do { if (RBUF_SPACE() > 2) SERIAL_UART_RTS_LO(); } while (0)
+    // prohibit to send
+    #define rbuf_check_rts_hi() do { if (RBUF_SPACE() <= 2) SERIAL_UART_RTS_HI(); } while (0)
+#else
+    #define rbuf_check_rts_lo()
+    #define rbuf_check_rts_hi()
+#endif
+
+
 void serial_init(void)
 {
     SERIAL_UART_INIT();
@@ -61,6 +77,7 @@ uint8_t serial_recv(void)
 
     data = rbuf[rbuf_tail];
     rbuf_tail = (rbuf_tail + 1) % RBUF_SIZE;
+    rbuf_check_rts_lo();
     return data;
 }
 
@@ -73,6 +90,7 @@ int16_t serial_recv2(void)
 
     data = rbuf[rbuf_tail];
     rbuf_tail = (rbuf_tail + 1) % RBUF_SIZE;
+    rbuf_check_rts_lo();
     return data;
 }
 
@@ -90,4 +108,5 @@ ISR(SERIAL_UART_RXD_VECT)
         rbuf[rbuf_head] = SERIAL_UART_DATA;
         rbuf_head = next;
     }
+    rbuf_check_rts_hi();
 }