PURPOSE: To prevent production of an invalid data by providing a storage means, a parallel/serial conversion means and a control means and applying parallel/serial conversion to the remaining data while skipping an invalid data when the invalid data exists in a parallel data.

CONSTITUTION: A parallel data inputted from n-set of signal lines is stored in a storage means 10. Then a parallel/serial conversion means 20 extracts the parallel data stored in the storage means 10 sequentially by using a data extraction pulse sent from a control means 30 and converts the extracted data into a serial data. When the means 30 recognizes the absence of data in (a+1)th and succeeding lines among n-sets of signal lines based on bit information, the (a+1)th-n-th pulses in the data extraction pulse generated repetitively in the lines 1-n are skipped. Then the 1st bit of the succeeding period is restored and the parallel/serial conversion is continued. Thus, production of an invalid data is prevented.