A method for measuring a deviation in timing of start of writing in scanning lines is implemented by an optical scanning device that scans a surface to be scanned with light beams from light-emitting elements having first and second light-emitting elements which are arranged so as to be displaced from each other in a direction corresponding to a direction of the scanning lines. The method includes: detecting timing at which light from the first light-emitting element is received by a light receiving element, as a first time, the light receiving element outputting a synchronization detecting signal before start of writing; detecting timing at which light from the second light-emitting element is received by the light receiving element, as a second time; and obtaining the deviation in timing for the second light-emitting element with respect to the first light-emitting element based on a difference between the first and second time.