An interface module comprises a serial signal transceiver to be connected to an external transmission path and an interface processing unit connected to the transceiver. The interface processing unit comprises a serializer/deserializer circuit, an encoder/decoder, a protocol processing unit having at least two kinds of selectable protocol processing functions, and a communicate mode switch circuit for changing reference clock to be supplied to the serializer/deserializer circuit in conjunction with switching of the protocol processing function from one to another.