An image forming apparatus (10) includes: a housing (60) in which a light source (60), a laser scanning member (62), a light detecting portion (66) configured to detect light incident on a predetermined position in a scanning path of the light scanned by the laser scanning member, and a scanning lens (64) configured to cause the light scanned by the laser scanning member (62) to be scanned at an equal speed on an image carrier (31), are disposed; a temperature gradient detecting portion (67), (68) configured to detect a temperature gradient of the housing (60), a heater (35) configured to heat the image carrier (31) or a sheet; and a correction processing portion (102) configured to correct the emission start timing based on the temperature detected by the temperature gradient detecting portion (67), (68) and a predetermined expression, and change content of the expression based on an operation state of the heater (35) at a start of an image formation process.