Abstract: A door closing device includes a cylinder body, a piston rod having one end extending out the cylinder body, and a locking unit including a casing, and a locking ring and an abutment member both disposed in the casing. The locking ring is disposed around the piston rod and is movable between inclined and non-inclined positions, where the locking ring engages and disengages the piston rod, respectively. A press unit is disposed on the casing and is movable between a pressed position, where the press unit presses the locking ring to move to the inclined position, and an unpressed position, where the press unit moves away from the locking ring. The abutment member abuts against the locking ring to keep the locking ring in the inclined position when the press unit is in the unpressed position.