A correlator includes a reference signal generating device for generating a reference signal with period T, n-convolvers each having an action time of T/n where n is an integer of at least 2, into each of which an information signal and a reference signal are input and each of which outputs a convolution signal of the information signal and the reference signal, a delay device for delaying an information signal input into a k-th convolver among the n convolvers for a time of (k-1)T/n where k=1, 2, . . . , n and for delaying a reference signal input into the k-th convolver for a time of (n-k)T/n, and an adder for adding convolution signals respectively output from the n convolvers. A receiver includes such a correlator for outputting a correlation signal of a received information signal and a reference signal, and a decoding circuit for decoding data from the correlation signal output from the correlator. A communication system includes a transmitter for transmitting an information signal, and a receiver for receiving the information signal transmitted from the transmitter and having the correlator.