A Mach-Zehnder optical waveguide (22) is formed immediately below a major surface (21A) of a substrate (21), and modulation electrode composed of a center electrode (24) and grounding electrodes (25-1, 25-2) are formed on the major surface (21A). The major surface (21A) of the substrate (21) is bonded to a base substrate (31) with a thermoplastic resin. The back (21B) of the substrate (21) is subjected to a predetermined machining to thin the whole substrate (21). The back (21B) of the thinned substrate (21) is subjected to laser machining or machining to form a thin portion (26). The base substrate (31) is separated from the substrate (21), the back (21B) of the substrate (21) is bonded to a major surface (27A) of a support substrate (27) having a groove portion (26) with a thermosetting resin in such a way that the optical guide (22) and the thin portion (26) are in the groove portion (26). Thus, an optical modulator (30) is manufactured.
