Apparatus for detecting the start and stop of an engine starter motor includes a sensor, an amplifier, a comparator, and a monostable multivibrator. When a starter motor starts and stops, magnetic flux around the starter motor varies greatly. The sensor senses this varying magnetic flux and produces a voltage signal corresponding to the magnetic flux variation. The voltage signal is amplified by the amplifier so that the amplified voltage signal is fed to the comparator. When the voltage signal fed to the comparator is greater than the reference voltage of the comparator, a pulse signal is fed to the multivibrator, and in turn the multivibrator is triggered to output detecting signals corresponding to the start or stop of the starter motor. A warning lamp or buzzer may be energized by the detecting signal from the multivibrator.