An optical modulation device (1, 2, 3) comprising: a substrate which extends in one direction; an optical waveguide (20, 21, 22) which is provided on the substrate in a longitudinal direction of the substrate; a half-wave plate; and a combining element (40A, 40B, 40C) which faces an end portion of the substrate and combines two types of linearly polarized light, which have vibration planes orthogonal to each other, to generate composite light, and an angle adjustment part which is provided between the end portion of the substrate and the combining element (40A, 40B, 40C), wherein the optical waveguide (20, 21, 22) modulates linearly polarized light, which propagates through an inside of the optical waveguide (20, 21, 22), to generate first polarized light and second polarized light which are linearly polarized light, the angle adjustment part adjusts incident angles of the first polarized light and the second polarized light with respect to the combining element (40A, 40B, 40C), the half-wave plate is provided at a position to which the second polarized light enters, the combining element (40A, 40B, 40C) includes a transparent base body, a first optical film which is provided on a first surface of the transparent base body, and a second optical film (412, 413, 414) which is provided on a second surface which faces the first surface of the transparent base body, the first optical film transmits one of the first polarized light and the second polarized light, which is emitted through the half-wave plate, and reflects the other light, the second optical film (412, 413, 414) reflects the one light, and the composite light is reflected in a direction intersecting the longitudinal direction. a substrate which extends in one direction; an optical waveguide (20, 21, 22) which is provided on the substrate in a longitudinal direction of the substrate; a half-wave plate; and a combining element (40A, 40B, 40C) which faces an end portion of the substrate and combines two types of linearly polarized light, which have vibration planes orthogonal to each other, to generate composite light, and an angle adjustment part which is provided between the end portion of the substrate and the combining element (40A, 40B, 40C), wherein the optical waveguide (20, 21, 22) modulates linearly polarized light, which propagates through an inside of the optical waveguide (20, 21, 22), to generate first polarized light and second polarized light which are linearly polarized light, the angle adjustment part adjusts incident angles of the first polarized light and the second polarized light with respect to the combining element (40A, 40B, 40C), the half-wave plate is provided at a position to which the second polarized light enters, the combining element (40A, 40B, 40C) includes a transparent base body, a first optical film which is provided on a first surface of the transparent base body, and a second optical film (412, 413, 414) which is provided on a second surface which faces the first surface of the transparent base body, a transparent base body, a first optical film which is provided on a first surface of the transparent base body, and a second optical film (412, 413, 414) which is provided on a second surface which faces the first surface of the transparent base body, the first optical film transmits one of the first polarized light and the second polarized light, which is emitted through the half-wave plate, and reflects the other light, the second optical film (412, 413, 414) reflects the one light, and the composite light is reflected in a direction intersecting the longitudinal direction.