PURPOSE: To avoid the transission of waveform distortion to a processor, by discriminating the waveform distortion in data signals from an external storage device, through the provision of a waveform distortion detecting circuit which prevents the transmission of abnormal data to an upper tank device.

CONSTITUTION: As to the waveform distortion detection for data signal, for example, if noise is included in an input data signal (a), a data delay signal (c) with a specified delay than the leading of noise of the data delay signal is outputted, a flip-flop 20 is triggered and a signal Q is outputted. The data signal (c) is given to an edge trigger flip-flop 13 via an inverter 12 to output a signal (i). The signals Q and (i) are given to a logical product circuit 14 to output a signal (j), and the signal (j) is given to a logical sum circuit 19, to output an error detection signal P. Thus, the data signal (a) is inputted to detect waveform distortion and noise is detected by inputting the data delay signal (c).