The present disclosure relates to an in-vehicle communication device and a time synchronization method thereof. The in-vehicle communication device includes a clock generator for generating a clock signal, a local counter for counting the number of pulses of the clock signal, a transceiver for receiving a message via an in-vehicle communication network, and a processor that determines whether the message received via the transceiver is a synchronization message, extracts synchronization time information from the synchronization message, and adjusts a count value of the local counter based on the synchronization time information to perform time synchronization.