To provide an image forming apparatus in which the assembling accuracy and assembling workability of a scanning optical device are excellent and to and from which the scanning optical device can be attached and detached.

In the scanning optical device 1, in the front end surface in the insertion direction of an optical case 21, a first insertion portion 101B obtained by defining a side surface as a reference surface and second and third insertion portions 101A and 101C obtained by defining a bottom surface as a reference surface are formed in the almost central portion and both end portions of the front end surface, respectively and in the rear end surface in the insertion direction of the optical case 21, a positioning portion 102 having a reference surface 102b and an engaging surface 102a is formed in the almost central portion. The attachment portion of an image forming apparatus body includes a front attachment means 200 having an apparatus body side reference surface coming into contact with the respective reference surfaces of the first, second and third insertion portions and a rear attachment means 300 having a portion to be engaged 301 where an apparatus body side reference surface 302 coming into contact with the reference surface of the positioning portion and an engaging portion are engaged with each other.