Herein disclosed is an optical modulator, having: a substrate (1); and a center and ground electrodes (4a to 4c), in which the substrate has ridge portions (8a to 8c), the center and ground electrodes are respectively formed above the ridge portions, the ridge portions below the center and ground electrodes respectively have top parts (10a, 10b) having a respective first and second end points (18, 19) separated with a distance of “WR”, the substrate has a bottom surface (21b) between the ridge portions having center and midway points (23, 24) positioned with a respective distance of WR/2 and WR/N (3≦N≦25) from the first end point, the ridge portion below the center electrode has a normal line (13), and the center point and the midway point define a straight line (25) crossed with the normal line at an angle larger or equal to 90.1°.