PURPOSE: To carry out the optimum start processing without an operator's mistaken judgment by defining diagnosis information on the main memory of a calculator system that has to ensure its continuity and start processing information and automatically making a diagnosis of the main memory at the time of the start processing of the calculator.

CONSTITUTION: A start control part 4 initializes the order (i) (i=1) of n- information areas and their start type (j) (j=1) in the main memory diagnosis information part 5. If the information area in the main memory diagnosis information part 5 is effective, that is, the i-th information area is present, initial images in the main memory 2 and those in submemory device 3 are compared by he specified sizes according to the main memory content comparison information of the main memory diagnosis information part 5. If the compared result shows that they are not the same images, that is, the main memory is destructed, the type of the start processing of the main memory diagnosis information part 5 is compared with the type of the worst start processing so far. If the current start processing is worse than the previous one, the type of the start processing is updated.