PURPOSE: To grasp engaging condition through a timing belt based on the detected value by detecting timing with a working point of an auxiliary device electrically with a position of the top dead center of a drive shaft such as a crank shaft as a reference.

CONSTITUTION: An adjusting means 15 is set in the center of an adjusting range of contact breaker to a rotating axis of a distributor 4 by a pulse motor 16. And a working signal of the distibutor 4 by the setting and a detecting signal of a top dead center sensor 8 are input in a detecting means 9 and compared to detect shift in engagement to a drive shaft and a driven shaft of a timing belt 5. And the detecting means displays the working signal and detecting signal on an analog meter. Then, it can be detected if the drive shaft 2, driven shaft 6 and the timing belt5 are engaged and connected with a proper timing relation.