PURPOSE: To prevent transient noise, etc., from being erroneously recognized as a control signal such as a chirp signal by deciding whether a polarity bit to be outputted from a polarity bit extracting circuit is coincident with a bit delayed by a delay circuit or not, and making a correlation degree ineffective when the polarity bits are coincident.

CONSTITUTION: The polarity bit of a received sound signal is delayed only for prescribed timing by delay circuits 46, (46-2)-(46-7). On the other hand, the polarity bit given with no-delay and the delayed polarity bit are supplied to a deciding circuit 52. At the deciding circuit 52, these two kinds of polarity bits are compared and when the result of comparison shows the polarity bits are coincident, the result of correlation degree calculation by comparing this polarity bit with a prescribed polarity pattern is made ineffective. Thus, the transient noise to be generated by rapid transition from large volume noise to a silent state, for example, can be prevented from being erroneously recognized as the control signal.