A data communication system comprises a transmitting device which transmits data and a receiving device which receives the data. The transmitting device comprises a clock generating circuit and a transmitting unit. The clock generating circuit generates a clock having a temperature characteristic in that a clock frequency varies with temperature. The transmitting unit transmits data generated in synchronization with the clock to the receiving device. The receiving device comprises a receiving unit, a detecting unit, a storage unit, and a calculating unit. The receiving unit receives the data. The detecting unit detects the clock frequency from the data. The storage unit stores temperature characteristic information regarding the temperature characteristic of the clock frequency. The calculating unit calculates a temperature corresponding to the clock frequency based on the clock frequency and the temperature characteristic information.