To provide a serial-parallel conversion circuit by which the content of output data of a synchronous serial data circuit can be measured in real time without problems even when a conventional slow measuring instrument is employed.

A selector 9 supplies a clock signal to only a shift register 3 or 5 on the basis of a selector signal. The shift register 3 or 5 which is supplied with the clock signal shifts fetched output data. The other shift register 5 or 3 which is not supplied with the clock signal holds the shifted data. On the basis of the selector signal, respective multiplexers 15 parallelly reads data at the output end of each stage of the shift register 5 or 3 which is not in shifting operation and supplies it to the measuring instrument.