To provide a device and a method for measuring a light intensity distribution highly accurately without using a magnification optical system.

A shielding plate 2 on which a fine light transmission part is formed and an optical sensor 1 are arranged movably over an LED (light emitting diode) head 3. A plurality of light emitting elements are arrayed in rows on the LED head, and the shielding plate 2 is moved in the row direction of the light emitting elements, and vibrated in the approximately orthogonal direction to the row direction of the light emitting elements by an actuator. The light intensity distribution in a direction approximately orthogonal to the row direction of the light emitting elements is measured by receiving by the optical sensor 1, light transmitted through the light transmission part formed on the shielding plate 2.