PURPOSE: To realize the extraction of a synchronizing signal for an MSK signal which can be produced through a simple constitution, by extracting the synchronizing signal out of the signal which is extracted at the middle of a COSTAS loop.

CONSTITUTION: An MSK signal sin(ωct±πt/2T) is branched into two parts and fed to the mixers 3a and 3b. The reproduced carrier signals of sinωct and cosωct are supplied to the mixers 3a and 3b from a voltage control oscillator 9. Thus cosπt/2T and ±sinπt/2T emerge at the outputs of the mixers 3a and 3b respectively. The outputs of the mixers 3a and 3b are multiplied through the squaring devices 19a and 19b respectively, and the difference output cosπt/T is used as a control signal of a phase synchronizing circuit 17.