PURPOSE: To attain completely the tracing function of a program by using a write control circuit which compares a sample data to be stored with another sample data stored in a preceding step and can prevent the writing when the coincidence is obtained between both data.

CONSTITUTION: A sample register 410 reads the storage data of a sample register 12 after receiving a trigger clock signal and delivers the data to a collating circuit after storage. When the same data are obtained continuously from a terminal 11, the circuit 420 detects the coincidence between both storage data of registers 12 and 410 and delivers it to AND circuits 432W434. An AND circuit 431 functions as a gate which transmits continuously the output pulse of a timing circuit 15 as a write command until a write inhibiting signal of a holding circuit 439 is delivered. For a series of actions of a write control circuit 430, a write command fed to a memory 14 is equal to the output pulse signal of the circuit 431. Then the circuit 439 works more than the prescribed number of times to prevent the output pulse signal. Thus no data is written.