A laser scanner comprises a laser driver which emits two different laser beams by one main-scanning line in correspondence with an input image signal, and two torsion mirrors to reflect the two different laser beams emitted from the laser beam emission means to scan the laser beams on an opposite photosensitive member. The two torsion mirrors conduct a reciprocal torsion movement in mutual opposite phases in synchronization with each other, and the laser beams reflected from the respective torsion mirrors are emitted alternately by one main-scanning line. Thus a laser scanner which enables image formation upon backward movement of the torsion mirror with a simple structure to increase the resolution and realize high-speed image formation, and an image forming apparatus to which the laser scanner is applied can be provided.