An optical coupler has a first terminal board and a second terminal board, a first conversion element for converting an electrical signal into an optical signal mounted on a surface of a first element mounting section of the first terminal board, a second conversion element for converting an optical signal into an electrical signal mounted on a surface of a second element mounting section of the second terminal board; and a light reflective curved surface formed so as to cover the first conversion element and the second conversion element. The surfaces of the first element mounting section and the second element mounting section have the same orientation. An optical signal emitted from the first conversion element is reflected on the light reflective curved surface, to optically couple the first conversion element and the second conversion element. The second conversion element has a plurality of light receiving cells that are electrically connected in series, arrayed in parallel with one another, and longer in a longitudinal direction than in a width direction. The first conversion element is arranged inside an area sandwiched between two contact lines that are set so as to be in contact with an outline of the second conversion element, and are in parallel with a length direction of the light receiving cells.