In a synchronous detection of an OFDM signal as an input signal composed of carrier waves, a clock generation part generates clock signals CK1 to CKk having a frequency quadruple of a frequency of each of carrier waves. A TAD sequentially calculates a moving average of the input signal by using the clock signals CK1 to CKk every quarter period of each carrier wave. A detection processing part sequentially executes addition-subtraction operation of the moving average values obtained by the TAD in order to calculate an amplitude value of each carrier wave. An unwanted component eliminating part subtracts amplitude values containing components of carrier waves having an odd multiple frequency of the carrier wave thereof from the amplitude values of the carrier waves in the input signal by a value which is an odd-submultiple of the amplitude value of the carrier wave having the odd-multiplied frequency.