To provide a data relay device which prevents a significant delay even when pieces of data are transmitted/received in protocols that are different from each other. Transmission timings at which pieces of data are transmitted/received in a network using an event trigger type communication protocol are estimated. Stay periods which occur when pieces of data are relayed and transmitted at predetermined transmission timings in a network using a time trigger type communication protocol are calculated while changing a reference time for the time trigger type communication protocol. Then, synchronization processing is performed in the network using the time trigger type communication protocol such that communication is performed using a reference time having a small stay period that has been calculated.