An image forming apparatus having an exposure optical system provided with an exposure light source in which light-emitting elements are arranged on a base plate to be line-shaped and with a lens array of a distributed refractive index type that converges a beam emitted from the exposure light source on the surface of a photoreceptor and conducts imagewise exposure. The apparatus is configured to satisfy the expression: ##EQU1## In the expression, .DELTA.T1 represents a variation width (K) of the temperature in the vicinity of light-emitting elements, R represents a radius of the photoreceptor mentioned above, Ri represents a length (.mu.m) of each member constituting the exposure optical system in the radial direction of the photoreceptor, .alpha. represents a coefficient of linear thermal expansion (.degree.C..sup.-1) of a transparent base that forms the photoreceptor, .alpha.i represents a coefficient of linear thermal expansion (.degree.C..sup.-1) of each member that constitute the exposure optical system, and F represents the depth of focus (.mu.m) of the lens array.