PURPOSE: To confirm a program while taking notice of the transition of the status, by storing data required for program confirmation in a storage device where the log of the execution of the program is stored when the transition of the status is detected.

CONSTITUTION: An operating device 1 sends address information to a storage device 2, and the device 2 sends instructions of a program, data of operations, etc. to the device 1. A program log storage controller 3 outputs required status data to a status data bus 15 and compares it with status data 14 of a log comparing register 5, where just preceding status data is stored, for the purpose of detecting the transition of the status. When the transition of the status is detected, a status transition detection signal 16 is outputted from the device 3, and contents of the register 5 are updated to status data of the bus 15, and status data of the bus 15 is stored additionally in a log storage device 4. Thus, log data for confirming the execution of the program is stored while taking notice of the transition of the status.