PURPOSE: To reduce the time for formation and the propagation loss of light by determining the crystal bearing of a semiconductor base plate and forming a V- groove by wet etching in the base plate and further squaring the groove by dry etching.

CONSTITUTION: SiO2 2 and a resist 3 are formed on a semiconductor base plate 1 of Si, and the resist in the part to be formed with a groove is removed by using a mask. The base plate is dipped in an etching soln. of (pyrocatechol + ethylenediamine + water), etc. to form a V-groove 5. Said groove 5 is dry etched by reactive ion etching (RIE), etc. to form the groove 5 into a square groove 6. Since the device for RIE is capable of accomplishing etching in a vertical direction at the speed several times higher than the speed of etching in a horizontal direction, said device can form the groove 6 in a shorter time. The resist 3 is thereafter removed and SiO2 to serve as a clad is formed on the inside surface of the groove 6, in which, for example, (SiO2 + Ti), etc. to serve as a core are packed to form the groove for an optical guide.