In a serial communication circuit, a data extracting section extracts reception data based on a reception clock signal with maximum speed. A pattern determining section compares a reception bit pattern of the reception data corresponding to a characteristic pattern and each of a plurality of detection bit patterns for the characteristic pattern, and indicates when the reception bit pattern matches one of the detection bit patterns. A periodicity determining section determines a period when the reception bit pattern matches the detection bit pattern, based on the pattern match indication, detects that the detection bit pattern emerges continuously in a stream of the reception data every the period, and determines a generation difference between transmission and reception speeds based on the detection bit pattern. A transmission rate setting section determines the transmission speed of a connected device transmitting the reception data based on the generation difference and maximum speed.