The present invention provides a sliding door opening/closing device for a vehicle that applies a sufficient opening/closing drive force to the left and right sliding doors and reduces a force necessary to lock and unlock the latch, despite a simple configuration of the device, and that facilitates the manufacturing process, improves operability and safety, and reduces noise. A lock device, against both sides of which locking portions abut, rotates a columnar permanent magnet so as to form magnetic locking circuits and fixes the locking portions by magnetic forces of the locking magnetic circuits. The rotational operation of the columnar permanent magnet is converted into the downward operation of a latch, and the lowered latch restrains the locking portions with respect to the lock device.