A light scanning device comprising: a light source which emits a laser beam; an optical unit including: a polygon mirror including n mirror surfaces, n being an integer equal to or larger than 1, and which guides the laser beam in a main scanning direction of an image bearing member; and a polygon motor including m poles, m being an integer equal to or larger than 1 and is coprime with n, and which rotates the polygon mirror; a signal output section which outputs a clock signal; and a light receiving section arranged on a start end side in the main scanning direction on a scanning path of the laser beam reflected by the polygon mirror, and which receives the laser beam and outputs a light reception signal; characterized in that the light scanning device further comprises: a clock-generation-number storing section which assigns serial numbers to periods obtained by dividing, by a number 3m/2, one cycle of the light reception signal periodically obtained by supplying the clock signal to the polygon motor to rotate the polygon mirror and stores, as a serial number corresponding to specific one surface among the n mirror surfaces, the serial number of at least one period among the divided periods synchronizing with edge timings of one of rising edges and falling edges of the clock signals in number required for rotating the polygon mirror once; and a specifying section which treats, as periods corresponding to the at least one period, periods equivalent to three consecutive serial numbers including the serial number of the at least one period and the serial numbers before and after the serial number, to specify, as a period in which the specific one surface among the mirror surfaces is used, a cycle of the light reception signal at the time when the edge timings are detected in the periods. a light source which emits a laser beam; an optical unit including: a polygon mirror including n mirror surfaces, n being an integer equal to or larger than 1, and which guides the laser beam in a main scanning direction of an image bearing member; and a polygon motor including m poles, m being an integer equal to or larger than 1 and is coprime with n, and which rotates the polygon mirror; a polygon mirror including n mirror surfaces, n being an integer equal to or larger than 1, and which guides the laser beam in a main scanning direction of an image bearing member; and a polygon motor including m poles, m being an integer equal to or larger than 1 and is coprime with n, and which rotates the polygon mirror; a signal output section which outputs a clock signal; and a light receiving section arranged on a start end side in the main scanning direction on a scanning path of the laser beam reflected by the polygon mirror, and which receives the laser beam and outputs a light reception signal; the light scanning device further comprises: a clock-generation-number storing section which assigns serial numbers to periods obtained by dividing, by a number 3m/2, one cycle of the light reception signal periodically obtained by supplying the clock signal to the polygon motor to rotate the polygon mirror and stores, as a serial number corresponding to specific one surface among the n mirror surfaces, the serial number of at least one period among the divided periods synchronizing with edge timings of one of rising edges and falling edges of the clock signals in number required for rotating the polygon mirror once; and a specifying section which treats, as periods corresponding to the at least one period, periods equivalent to three consecutive serial numbers including the serial number of the at least one period and the serial numbers before and after the serial number, to specify, as a period in which the specific one surface among the mirror surfaces is used, a cycle of the light reception signal at the time when the edge timings are detected in the periods. a clock-generation-number storing section which assigns serial numbers to periods obtained by dividing, by a number 3m/2, one cycle of the light reception signal periodically obtained by supplying the clock signal to the polygon motor to rotate the polygon mirror and stores, as a serial number corresponding to specific one surface among the n mirror surfaces, the serial number of at least one period among the divided periods synchronizing with edge timings of one of rising edges and falling edges of the clock signals in number required for rotating the polygon mirror once; and a specifying section which treats, as periods corresponding to the at least one period, periods equivalent to three consecutive serial numbers including the serial number of the at least one period and the serial numbers before and after the serial number, to specify, as a period in which the specific one surface among the mirror surfaces is used, a cycle of the light reception signal at the time when the edge timings are detected in the periods.