PURPOSE: To initialize or erase only the RAM memory data of an electronic control device for an internal combustion engine without erasing the memory data of other electronic devices by providing a failure diagnosing tester which can be used by connecting to a control circuit.

CONSTITUTION: A code for required diagnosis-object equipment or for initializing a RAM from a tester 27 is received by a control circuit 25. Then, the control circuit 25 judges which code it has received and, at the time of the diagnosis- object equipment code, the result of diagnosis of the diagnosis-object equipment is outputted to the tester 27 while, at the time of the RAM initializing code, the initialization or erasing of the RAM is executed in accordance with the control program of a ROM. Thereby, the initialization of the RAM can be carried out without need for cutting off a feeding voltage from a storage battery, thereby, without erasing the memory data in other electronic equipment loaded on a vehicle.