An optical writing device that forms an electrostatic latent image on a photoreceptor, the optical writing device comprising: a pixel-information acquiring unit that acquires pixel information that is information about pixels that form an image as the electrostatic latent image; a line-pixel-information storage unit that stores the acquired pixel information corresponding to a plurality of lines for each main scanning line; a light source device that exposes the photoreceptor to a light to form the electrostatic latent image, the exposure with the light being performed with a period that corresponds to an N-fold sub-scanning direction resolution, the N-fold sub-scanning direction resolution being an integral multiple of a sub-scanning direction resolution of the pixel information; a positional-shift-information storage unit that stores a positional shift information in a sub-scanning direction that differs depending on a position in a main-scanning direction of the light source device; a sub-line counting unit that counts a sub-line period that corresponds to the N-fold sub-scanning direction resolution; a reading-line selecting unit that selects a main scanning line, as the main scanning line from which the pixel information is read out, that differs depending on the position in the main scanning direction, on the basis of a calculated value from the counted value of the sub-line period and the positional shift information; and a pixel-information reading unit that reads out the pixel information from the selected main scanning line and inputs the read pixel information to the light source device, wherein the reading-line selecting unit performs N times of selection that determines which main scanning line is to be selected as the main scanning line from which the pixel information is read out, wherein N is the n-th power, which is an integer power, of two, characterised in that the reading-line selecting unit performs the N times of selection that determines which main scanning line is selected as the main scanning line from which the pixel information is read out by switching the main scanning line selected as the main scanning line from which the pixel information is read out every time when the sub-line period is counted N times, and the reading-line selecting unit detects whether the sub-line period is counted N times every time when a lower n-bits count value of a counter becomes zero, the counter that counts the sub-line period at the sub-line counting unit. a pixel-information acquiring unit that acquires pixel information that is information about pixels that form an image as the electrostatic latent image; a line-pixel-information storage unit that stores the acquired pixel information corresponding to a plurality of lines for each main scanning line; a light source device that exposes the photoreceptor to a light to form the electrostatic latent image, the exposure with the light being performed with a period that corresponds to an N-fold sub-scanning direction resolution, the N-fold sub-scanning direction resolution being an integral multiple of a sub-scanning direction resolution of the pixel information; a positional-shift-information storage unit that stores a positional shift information in a sub-scanning direction that differs depending on a position in a main-scanning direction of the light source device; a sub-line counting unit that counts a sub-line period that corresponds to the N-fold sub-scanning direction resolution; a reading-line selecting unit that selects a main scanning line, as the main scanning line from which the pixel information is read out, that differs depending on the position in the main scanning direction, on the basis of a calculated value from the counted value of the sub-line period and the positional shift information; and a pixel-information reading unit that reads out the pixel information from the selected main scanning line and inputs the read pixel information to the light source device, wherein the reading-line selecting unit performs N times of selection that determines which main scanning line is to be selected as the main scanning line from which the pixel information is read out, wherein N is the n-th power, which is an integer power, of two, characterised in that the reading-line selecting unit performs the N times of selection that determines which main scanning line is selected as the main scanning line from which the pixel information is read out by switching the main scanning line selected as the main scanning line from which the pixel information is read out every time when the sub-line period is counted N times, and the reading-line selecting unit detects whether the sub-line period is counted N times every time when a lower n-bits count value of a counter becomes zero, the counter that counts the sub-line period at the sub-line counting unit.