PURPOSE: To miniaturize a laser scanning optical device.

CONSTITUTION: The device is provided with a laser light source 1 for emitting the laser light L, a rotary polygon mirror 3 for scanning with the laser light L in a main scanning direction, a condensing lens 2B for converging the laser light L in a subscanning direction, an f-θ lens 4 for almost constantly condensing the scanned laser light L on a plane to be scanned, a horizontal sync. detecting sensor 6 for receiving the laser light L as the end part of the scanning extent of the mirror 3, and a horizontal sync. detecting lens 5 for condensing the laser light L made incident on the sensor 6, and the horizontal sync. detecting lens 5 is arranged not near the f-θ lens 4 but near the rotary polygon mirror 3.