an fsk signal modulator is provided in a transmitter which receives desired information to be transmitted on its input and which modulates the information to be transmitted to transmit a binary fsk signal . a counter counts a value of addition with the value of addition modified in accordance with a predetermined rule , depending on the value specified by the information to be transmitted , and for holding the counted value . the count value is determined by a threshold value decision circuit with respect to a threshold value . the result from the decision is output in the form of binary fsk signal . an fsk signal modulator will be provided which is simplified in circuit constitution .