A communication system is provided which is capable of easily setting a transmission speed between a signal transmitter and a signal receiver to carry out information communication. A transmitting device transmits one frame of measuring data which contains a start bit to be added to a head of the data and a stop bit to be added to an end of the data and which is used for a signal receiver to measure a transmission speed. A framing error detector in a receiving device receives the measuring data for detection, at every measuring point, of a framing error which occurs when a transmission speed of the signal transmitter does not coincide with a transmission speed of the signal receiver and normal detection of a stop bit is impossible and generates information about detection of a framing error. A transmission speed measurer measures a transmission speed of the transmitting device based on information about detection of a framing error and measuring point interval time.