PURPOSE: To attain clock changeover without causing malfunction of a logic circuit in an equipment by providing a 2nd frame pulse selection means and a latch means to the pre-stage of a system changeover frame pulse generating means.

CONSTITUTION: The circuit is provided with a 2nd frame pulse selection means 3 selecting and outputting the frame pulse of a current system among frame pulses outputted from 1st and 2nd clock sources 1, 2 by an external system changeover command signal, and a latch means 4 latching the external system changeover command signal from the output of the 2nd frame pulse selection means 2 and outputting a system selection signal. The 2nd frame pulse selection means 3 and the latch means 4 latch the system selection command signal inputted from a frame pulse generating means 22 used at the time of changing over the system till the fall of a frame pulse signal from a clock source made to the active system newly after system changeover. Thus, malfunction of a logic circuit in an equipment at system changeover is prevented.