Monitoring system for maintenance of cars used for the purpose of automatically informing time for replacement of objects to be replaced, such as oil, filters etc. Said monitoring system includes a detector of engine operation for detecting operating time of cars (1), a variety of switches (3, 4, 5, 6) and sensors (7, 8, 9), a device for storing predetermined time of replacement of each object to be replaced (13), a device which deducts said operating time from said predetermined time of replacement and outputs the result of the deduction as a signal (12), and a multi-display (2) for displaying objects to be replaced on the basis of said output signal. In such a manner, operators or persons in charge of cars can surely notice the time of replacement of each object to be replaced by means of the information on the display (2); therefore, said object can be replaced at an optimum time for replacement without replacement time unnoticed.