A communication device comprising: a LINK configured to perform protocol conversion of a signal from a Master and output the converted signal to a Slave SerDes, and perform protocol conversion of a signal from the Slave SerDes and output the converted signal to the Master, wherein the LINK is capable of alternatively selecting a first mode and a second mode when transmitting the signal from the Master to the Slave SerDes, in the first mode, the LINK repeats processing of converting a 1-byte signal transmitted from the Master into a signal of a first communication standard in units of the 1-byte signal, transmits the converted signal to the Slave SerDes, then receives a signal of the first communication standard including an ACK signal indicating an acknowledgement or a NACK signal indicating a negative acknowledgement, converts the received signal into a signal of a second communication standard, and transmits the converted signal to the Master, and in the second mode, the LINK transmits a signal including the ACK signal or the NACK signal to the Master each time receiving a multi-byte signal transmitted from the Master byte by byte, collectively transmits converted signals to the Slave SerDes after the conversion of the multi-byte signal received from the Master is completed, then receives and retains the signal of the first communication standard including the ACK signal or the NACK signal from the Slave SerDes, and then converts the signal of the first communication standard into a signal of the second communication standard and transmits the converted signal to the Master in response to a readout request from the Master, and the signal transmitted to the Slave SerDes includes command information indicating content transmitted from the Master, and the signal transmitted to the Master includes command information indicating content transmitted from the Slave SerDes. a LINK configured to perform protocol conversion of a signal from a Master and output the converted signal to a Slave SerDes, and perform protocol conversion of a signal from the Slave SerDes and output the converted signal to the Master, wherein the LINK is capable of alternatively selecting a first mode and a second mode when transmitting the signal from the Master to the Slave SerDes, in the first mode, the LINK repeats processing of converting a 1-byte signal transmitted from the Master into a signal of a first communication standard in units of the 1-byte signal, transmits the converted signal to the Slave SerDes, then receives a signal of the first communication standard including an ACK signal indicating an acknowledgement or a NACK signal indicating a negative acknowledgement, converts the received signal into a signal of a second communication standard, and transmits the converted signal to the Master, and in the second mode, the LINK transmits a signal including the ACK signal or the NACK signal to the Master each time receiving a multi-byte signal transmitted from the Master byte by byte, collectively transmits converted signals to the Slave SerDes after the conversion of the multi-byte signal received from the Master is completed, then receives and retains the signal of the first communication standard including the ACK signal or the NACK signal from the Slave SerDes, and then converts the signal of the first communication standard into a signal of the second communication standard and transmits the converted signal to the Master in response to a readout request from the Master, and the signal transmitted to the Slave SerDes includes command information indicating content transmitted from the Master, and the signal transmitted to the Master includes command information indicating content transmitted from the Slave SerDes.