In a condition monitor system for controlling a power supply in a vehicle, a charge/discharge circuit has a capacitor which is discharged in response to each of speed pulses of a speed sensor and periodic pulses of a microcomputer. When none of the pulses are generated due to abnormality, the capacitor is continuously charged without being discharged. When the charge voltage of the capacitor reaches a switching voltage, a relay is turned off to shut off the power supply to various parts in the vehicle. The charge/discharge circuit is commonly used to check generation of the speed pulses and the periodic pulses by setting the switching voltage in accordance with a possible maximum time interval of the speed pulses.