A side sliding door device is provided for a vehicle that opens and closes an entrance on a side of the vehicle by a sliding door movably supported on a horizontal door rail. The sliding door device includes an automatic locking mechanism that is interlocked with a closing operation of the sliding door, and locks the sliding door in a closed state, and a manual locking mechanism that locks the sliding door in the closed state by a manual operation. The automatic locking mechanism and the manual locking mechanism are secured on a common base plate on a vehicle body and integrated into one unit, so that the manual locking mechanism prevents an unlocking operation of the automatic locking mechanism in a locked state and holds the sliding door in the locked state.