PURPOSE: To perform a clock switching action with no overlap nor omission by producing a selection signal synchronizing with own clock after confirming the non-selection of other clocks to own clock selection signal and selecting own clock.

CONSTITUTION: The clock signals CLK.AWC which are not synchronous with each other are switched by a selection output circuit 7 according to the selection signal applied to a decoder 8. When the signal CLK.A, for example, in selected, a selection signal SEL.A is selectable with signals SEL.B and C nonselectable respectively for the output of the decoder 8. A selection designating part 5-1 judges the outputs of synchronizing circuits 6-2 and 6-3 and confirms the non- selection of those outputs to send the selection signal SEL.A to a synchronizing circuit 6-1. This circuit 6-1 consists of shift registers A0WA2 which are shifted by a clock A. Thus the signal SEL.A for the clock A is synchronized with the clock A and applied to the circuit 7.