PURPOSE: To provide convenience in operational management of a robot system by erasing the initially registered contents when a read part commences registration in the condition that the memory part has no empty area, and transferring the registered contents of each memory area having a No. to another memory area having No. one smaller than the previous.
CONSTITUTION: When operation is commenced in the condition that all area of a memory 6 are unoccupied, the contents of the signal being entered or emitted at the current time are at regular intervals registered one after another from the first No. memory area. If operation continues even after data are registered in all memory areas, the memory control part 8 is thereafter operated each time prior to actuation of the read part 7, to erase the registered contents of the first no. memory area and transfer the registered contents of the other No. memory areas to respective memory areas having one smaller No. This permits knowing the past operating situations within a certain range of time backward from the present and eventual variation of the situation.