To provide an image forming device capable of providing a color image without color shift and an optical scanner suitable for the image forming device.

Though ideal distance Y (mm) between an angle θ (radian) formed by optical axes and incident light fluxes of f θ lens systems 5, 6 and an optical axis of a reached point of the light fluxes on a photosensitive drum 8 is K.θ (K: f θ coefficient), actual distance Y' to reach the photosensitive drum 8 has an error (f θ error) against the ideal distance Y due to aberration and tolerance. Then, for example, f θ coefficients K are defined as =329.03 (mm) and 327.76 (mm) respectively against, for example, a light flux with incident angle = zero degree from a semiconductor laser 1' and a light flux with incident angle = four degrees from the semiconductor laser 1' and the f θerrors of the light fluxes with incident angle = zero degree and incident angle = four degrees are made to be approximately zero. To vary the f θ coefficients K, magnification of image data to be written or time (pixel clock) equivalent to one pixel is changed.