PURPOSE:To make possible the assembling of an optical integrated circuit that can be connected to core layers having several mum thicknesses by forming optical elements having optical guides of equal thickness and height to the cores of optical fibers consisting of clad layers and cores disposed on a substrate. CONSTITUTION:An LiNbO3 crystal plate 2 having the thickness equal to the sum of the thicknesses of the clad layer 15 and core layer 14 of an optical fiber 5 is mounted on a substrate 1, and an optical guide 9 having the thickness equal to that of the core layer 14 is formed by diffusing Ti on the surface thereof, and comb-type electrodes 18 are provided on the surface thereof. Thick film lenses 3, 4 are formed on both sides of the optical guides 9 in a manner as to have optical guides 10, 11 of slightly larger refractive indexes by an ion exchange method. The clad layers 12 deposited by a CVD method and the core layers 12 formed by coating and setting of a transparent plastic layer are formed on both sides thereof, and the entire surface is covered with a clad layer 16 of plastics, whereby the optical fibers 5, 6 are connected. Thereby, the optical integrated circuit that can be connected to slender cores 14 is formed.