An optical device includes a semiconductor optical modulator element (46) which is connected to an end of a signal line (42) in form of a microstrip high-frequency line and whose other electrode is connected to a ground electrode (48) by a bonding wire (50). The signal line (42) includes first and second line portions (42a, 42b). The first line portion (42a) nearer to the entrance of a high-frequency signal has a width W1 determined to adjust its characteristic impedance to 50.OMEGA. and a length L1 equal to an integer multiple of 1/4 of its own guide wavelength. The second line portion (42b) has a width W2 determined to adjust its impedance to an intermediate value between the impedance of the optical modulator element 46 and the impedance of the first line portion (42a), and a length L2 equal to an integer multiple of 1/4 of its own guide wavelength.