PURPOSE: To detect a single frequency sinusoidal wave stably by providing a single frequency sinusoidal wave generator, two multipliers, two low pass filters, two square devices and one adder.

CONSTITUTION: A single frequency sinusoidal wave generator 2 whose 2 outputs 3, 4 have a phase difference of 90° is provided to the system. A reception signal from an input terminal 1 and a 1st output 3 of the single frequency sinusoidal wave generator 2 are multiplied by a multiplier 5, the result passes through a low pass filter 7 and its output is squared by a square device 9. Moreover, the reception signal and a 2nd output 4 of the single frequency sinusoidal wave generator 2 are multiplied by a multiplier 6, the result passes through a low pass filter 8 and its output is squared by a square device 10. Then the output of the square device 9 and the output of the square device 10 are added by an adder 11 and its output is measured by a level discriminator 12. Thus, the presence of a sinusoidal wave desired to be detected is stably detected.