PURPOSE: To attain the entire cost-down including manufacture by connecting an impedance element between the input of a phase shift means and a reference potential then making the input frequency whose detected output goes to zero coincident with the matching frequency which minimizes the distortion.

CONSTITUTION: A phase shift means 2 phase-shifting the input FM signal by 90° and generating an output having a phase change in response to the frequency shift of the input FM signal is provided, the input FM signal and the output of the phase shift means 2 are multiplied to use the low frequency component as the FM detection output, and an impedance element 9 is connected between the input of the phase shift means 2 and the reference potential. For example, a capacitor 9 is connected between the input of the phase shift circuit 2 and the ground, and in selecting the capacitance Co' of the capacitor 9 to be 1/ωo2Lo, the phase is coincident with 90° at the input frequency at the time of matching, to minimize distortion, thereby reducing the cost.