A photoelectric monitor for a mold comprises a pair of securement members secured to opposite sides of a molding zone of a mold and provided with holes which are matched one-to-one with each other across the molding zone, two sensor blocks secured to a molder as spaced apart from the mold and including light-emitting elements and light-receiving elements, a controller connected to each of the sensor blocks, optical fibers each having its one end detachably connected to the light-emitting element of one of the sensor blocks and its other end detachably connected to one of the securement members through the hole, and optical fibers each having its one end detachably connected to the light-receiving element of the other sensor block and its other end detachably connected to the other securement member through the hole.