A data signal is transmitted synchronously with a clock signal, and contains n phases (n represents an integer of 2 or more) of data for each cycle of the clock signal. A first time to digital converter generates clock change point information which represents the change timing of the clock signal. A second time to digital converter receives a data sequence in increments of cycles of the clock signal, and generates data change point information items which represent the change timing of the data in increments of phases of the data. A calculation unit calculates difference data between the change timing represented by the data change point information and the change point timing represented by the clock change point information in increments of phases. A judgment unit judges a DUT based upon the difference data received from the calculation unit.