PURPOSE: To make it possible to confirm an unreceivable key which can not be received because of no relation with a flow being operated at present or a failed key by comparing and collating a key code read out from a memory area in accordance with a test program with a depressed key code.

CONSTITUTION: In the case of testing a key switch of operator's port key switches 104, codes corresponding to keys successively depressed by an operator are read out from a memory and sent to a control part 11. The control part 11 compares a received code with a code read out from a corresponding address in a memory area to be used for the operation of the test program, and when both the codes coincide with each other, advances the test to the succeeding key, and if the coincidence of key codes is not obtained until a timer started simultaneously with the start of a test for each key is over, judges the tested key as a failed one. Which substitutive key is used for registering the function of the failed key is inquired and the code of the failed key is stored in the address of the memory area corresponding to the substitutive key.