To omit a power supply circuit by receiving power from a signal line used for controlling data transmission, to conduct proper data transmission and to reduce the communication time by improving the efficiency of data transmission.

A basic unit of transmission data TXD and reception data RXD is composed of a status S and a command C in two bytes and data D in one byte. The status S indicates a status of the data D, e.g. kind of information of the data D and its order. The command C indicates the processing operation of the communication opposite party. Furthermore, the actual transmission data TXD and reception data RXD are configured by combining a plurality of three-byte information units. Since part of time information such as hour, minute and second is designated by using the status S, when part of it is desired to be changed, it is not required to send all time information so as to shorten the communication time.