PURPOSE: To obtain a serial-parallel converting circuit which reduces the load on a microprocessor with small hardware by sampling bits in the center of each bit through a bit sampling circuit provided in front of the microprocessor.

CONSTITUTION: The serial-parallel converting circuit is an external circuit including flip-flops 1 and 3 and a counter 4, and this circuit generates an input request signal outputted by an interrupt signal synchronizing with every one-bit length; and the microprocessor 8 inputs a bit serial signal 8 synchronously with this input request signal and error testing, serial-parallel conversion, etc., are carried out thereafter by software. The addition of the external circuit eliminates the need for the micrprocessor 5 to input the bit serial signal 8 by sampling, and the bit serial signal is inputted only once for each bit, so the load on the software of the microprocessor 5 is reduced.