PURPOSE: To diagnose the quality of static compressive force accurately for every cylinder of an engine, by analyzing a current flowing a starter.

CONSTITUTION: A data processor 10 receives a voltage signal of a battery 1 from a voltage sensor 11; a current signal, which flows into the starter from a current sensor 12; a low-voltage side signal from a coil voltage sensor 13; and a No.1 cylinder plug signal from a reference cylinder sensor 14. The processor 10 performs data processing. During the test, cranking of the engine is continued, and the current flowing the starter is mainly processed. The respective peak value of the starter current corresponds to a driving current required for the starter 3 when each cylinder is compressed. Therefore, the static compressive force of the engine can be found by analyzing the peak value for each cylinder.