PURPOSE: To reduce a shift of a light spot position by using a polyhedral rotary mirror of a multiple structure consttituted of plural polyhedral mirrors.

CONSTITUTION: A polyhedral rotary mirror 7 is formed by sticking two polyhedral mirrors 12, 13 with a difference of a phase angle α, and two beams 21, 22 are scanned with a time difference Δt. Δt=α/ω, provided that ω is an angular velocity of the polyhedral rotary mirror 7. In this state, if a variation occurs in a revolving speed of the polyhedral rotary mirror 7, when a set value or a design value of the time difference Δt and the angular velocity ω are deonted as Δtd and ωd, respectively, and those which are caused by a variation of the rotation are denoted as Δtc and ωc, respectively, a shift Δx of a light spot position after T seconds from the scanning start point is shown by Δx=(vd-vc)T=2FαT(1/Δtd-1/Δtc). Provided that vd and vc denote a set value or a design value of an optical scanning speed, and that which is caused by a variation of the rotation, respectively, and F denotes a focal distance of an Fθ lens. When a design value of a time T extending from a scanning start point to a recording point, and that of a time of a variation of the rotation are denoted as Td and Tc, respectively, Δx=vdTd-vcTc=2Fα(Td/Δtd-Tc/Δtc) is obtained, and the time T can be varied by changing a picture signal click, and the sift Δx can be reduced.