When data has been received by a sending and reception unit, a transmission control unit generates an ACK and outputs it to an ACK reduction unit. A reception interval measuring unit detects a time interval in the data which is received, and starts ACK reduction process by the ACK reduction unit if this time interval is greater than or equals to a first predetermined value. In this case, the ACK reduction unit reduces the number of ACKs which are transmitted via the sending and reception unit. An ACK interval measuring unit detects a time interval of generating ACKs by the transmission control unit, and stops the ACK reduction process by the ACK reduction unit if this time interval is greater than or equals to a second predetermined value.