Optical waveguide and the method for making the same, in which a semiconductor laser diode and an optical isolator are formed on one substrate in a monolithic manner. At the moment, a semiconductor laser diode is made by a III-V compound semiconductor crystal, such as GaAs, InP, etc., and an optical isolator is made of iron garnet to fabricate these in monolithic, and this invention was established by developing the technology to monocrystallize at least a small part of the garnet film containing no iron which is deposited on the semiconductor substrate and to use this as the seed crystal for the said phase epitaxial growth of the iron garnet film which forms the iron garnet optical waveguide.