The present invention relates to a serial printer. 
In the prior art, a serial printer using a microprocessor is equipped with a watchdog timer as a countermeasure against runaway of the microprocessor.  This watchdog timer is programmed to be cleared at a certain frequency, when the microprocessor is in its normal operation, but not when the microprocessor runs away.  When a predetermined value is reached, a malfunction is detected so that the microprocessor is reset. 
The watchdog timer of the existing serial printer is cleared in response to a clearing instruction of the program when the microprocessor is in its normal operation.  When the number of clearing instructions is small, the watchdog timer reaches the predetermined value even in the normal operation of the microprocessor.  Then, this microprocessor may be reset.  When the number of clearing instructions is large, on the contrary, the watchdog timer may be cleared so as not to reset the microprocessor even if the microprocessor runs away.  Thus, it is difficult to time the generation of clearing instructions in the program.  This difficulty makes it impossible to say that run-away of the microprocessor will be halted without fail.