A side sliding door apparatus for a vehicle includes one of two sliding doors driven by an actuator, and the other moves in conjunction with the one sliding door via a direction converting mechanism. When the sliding door is in a closed condition, a latch rod is inserted into a lock hole to lock the sliding doors in the closed condition. A roller is pushed up on a cam surface when the sliding doors are opened, which releases the locking. An upper level surface of the cam surface forms an inclined surface with a downward gradient, which prevents the roller from unexpectedly going beyond the starting end of the upper level surface and dropping due to vibration or the like being applied to the slider in the unlocked condition, and prevents the locking with the locking mechanism in response to the next sliding door closing command.