This invention discloses a monostable multivibrator which is useful for an FM detector circuit of a pulse count system. The monostable multivibrator has a time constant circuit which includes a capacitor, an amplifier circuit which receives an output of the time constant circuit, a positive feedback circuit which is connected between an output end of the amplifier circuit and an input end of the time constant circuit, and a trigger terminal which is disposed in a circuit loop constructed of the time constant circuit, the amplifier circuit and the positive feedback circuit; and is characterized in that the amplifier circuit is a differential amplifier which is made up of a pair of transistors connected in the differential form, the transistors being connected in common through emitter resistances connected in series with respective emitters thereof. Thus, the monostable multivibrator can provide pulse signals of a fixed pulse width without being influenced by noise.