Abstract: A slingshot type spear gun for underwater fishing comprises a rod, a guide for an arrow at the forward end of the rod, and a pistol grip trigger mechanism at the rear end of the rod. A sleeve slidably mounted on the rod has one end of the rubber bands used for propelling the arrow fastened thereto, one pair of cross bars extending sidewise, and a mechanism for automatically releasably locking the sleeve to the rod. In order to cock the gun, the sleeve is pulled backwardly along the rod to a position in which the yoke at the free ends of the rubber bands may be easily engaged in a notch on the arrow. Thereafter, the sleeve is shifted forwardly along the rod by the user, pushing it with the feet bearing on the cross bars, until it reaches the desired loading degree of the rubber bands. The locking mechanism will automatically lock the sleeve in the desired position, against the action of the rubber bands.