PURPOSE: To simplify a measuring work by a method wherein the timing light is irradiated when a revolving number or the negative pressure of a suction pipe has arrived at a standard value, in the timing light or the like measuring a spark-advance in the ignition device of an internal-combustion engine.

CONSTITUTION: The revolving number or the negative pressure of the suction pipe is detected by first detector 1. The detecting signal is compared with the signal of a setter 2 setting a standard value. The second detector 4 is a plug ignition sensor clipped to the ignition plug chord of a first cylinder, for example. The timing light 6 is irradiated by a gate circuit 5 when both signals from a decision circuit 3 and the second detector 4 are generated together. According to this method, the measuring work may be simplified.