In a wireless communication system, a portable device transmits a first signal spread based on a reference period indicated by a synchronization signal transmitted from an in-vehicle device and transmits a second signal spread based on an operation by a user. The in-vehicle device sets a search period based on a variation range in a delay time from when the synchronization signal is transmitted to when the spread first signal is transmitted and sets a residual period that starts at an ending point of the search period and ends at an ending point of the reference period when a starting point of the reference period is set at a starting point of the search period. When the in-vehicle device fails in a synchronous acquisition for the search period, the in-vehicle device performs a synchronous acquisition process for the residual period.