To provide a spot size converter for reducing coupling loss between optical elements and reducing time required for assembling work without aligning the optical elements at the time of module assembly, an optical coupling element and an optical element module.

In photonic crystals 1 and 11 composed by forming a cycle structure that two or more kinds of the media of different refractive indexes have a photonic band gap, a waveguide formed so as not to satisfy the array of the cycle structure is provided so that light having a frequency within the photonic band gap area of the photonic crystal is passed through. For the waveguide 1b, at least one of the cross section size and cross section shape of the waveguide is changed inside the waveguide and the spot diameter of the light emitted from the emission port of the waveguide is turned to a size equal to or more than the wavelength of the light.