PURPOSE: To facilitate the maintenance, the diagnosis, and the debugging processing of a device to be observed, by starting the tracing operation when a preliminarily set condition is detected and continuing the tracing opration for a preliminarily set certain time.

CONSTITUTION: When the trace start condition is detected by a trace start condition detecting circuit 150, a controlling circuit 120 initializes a counting circuit 160 and prepares for the start of the tracing opration. Hereafter, each time the trace condition is detected by a trace condition detecting circuit 110, contents of an address register 140 and the circuit 160 are increased by one and updated. Simultaneously, state information from the device to be observed is written in the address, which is designated by a register 140, of a storage circuit 130. When the value of the circuit 160 reaches a certain value, the circuit 120 suppresses immediately the generation of the write signal to the circuit 130 and stops the update of contents of the register 140 to terminate the tracing operation. Thereafter, each time the trace start condition is detected, said operation is repeated.