PURPOSE: To easily detect the error of a signal and to display it as an error flag by extracting a period which is assumed as the error as the pulse period of an error detection permission signal when the fluctuation of a data occurs.

CONSTITUTION: An edge detection circuit 11 detects level change generated in a shift clock 14 after passing a buffer 6, and outputs an edge detection signal 15. When the signal 15 is generated in the pulse period when the error permission signal represents a permission state, the information of the fact is held with the error flag 12, and the fact that a noise is generated in the shift clock and the mis-shift of data occurs is displayed. Thereby, the presence/absence of the error in a reception signal can be easily detected.