Disclosed is a correlation method to adaptively operate in a time varying phase and a correlator thereof. A correlation method according to the present invention includes performing the first correlation according to a certain reference value and a certain reference interval for correlation of a received signal; calculating an average variation of the received signal through the result of performing the first correlation; deciding a new reference value for the second correlation through the average variation; performing the second correlation according to the new reference value and the reference interval; and selecting the maximum value among the result of performing the second correlation. Accordingly, plural references are decided considering variation of a response signal between tags and also separate plural references are decided considering bit variation of a response signal in the same tag, so that a digital signal can be ideally demodulated in the environment where signal phase variation occurs.