An optical rotary encoder (1; 51; 83; 94; 100) comprising: a rotating disc (3; 53; 103) provided with a plurality of detection tracks that are formed concentrically; a light-emitting element (2; 52; 101) that irradiates the detection tracks with detection light (7; 57; 102); a fixed slit plate (4; 54; 105) provided with a slit pattern (54a; 107) that allows passage of optical signals obtained from regions irradiated with the detection light (7; 57; 102) in each of the detection tracks; and a light-receiving element (5; 55; 104) that receives each of the optical signals that have passed through the slit pattern (54a; 107), wherein each of the detection tracks is configured from a plurality of slits or reflective zones arranged at regular angular intervals; the slit pattern (54a; 107) in the fixed slit plate (4; 54; 105) is configured from slit rows, the number of slit rows corresponding to the number of detection tracks; the slit rows include a plurality of first slit rows (21, 23, 26) and a plurality of second slit rows (22, 24, 25), the first and second slit rows being lined up in the radial direction (y) of the rotating disc (3; 53; 103); the slits (21a, 23a, 26a) of the first slit rows (21, 23, 26) are arranged at an interval in the circumferential direction that is shorter than the interval of the second slit rows (22, 24, 25); and the slit pattern (54a; 107) is formed so that those slit rows (21, 26) that are located at an innermost and at an outermost position in the radial direction (y) of the rotating disc belong to the first slit rows (21, 23, 26), the radial direction (y) being a direction in which the slit rows are lined up, and the slit pattern (54a; 107) being positioned within an effective irradiation region of the detection light (7; 57; 102) in the fixed slit plate (4; 54; 105). a rotating disc (3; 53; 103) provided with a plurality of detection tracks that are formed concentrically; a light-emitting element (2; 52; 101) that irradiates the detection tracks with detection light (7; 57; 102); a fixed slit plate (4; 54; 105) provided with a slit pattern (54a; 107) that allows passage of optical signals obtained from regions irradiated with the detection light (7; 57; 102) in each of the detection tracks; and a light-receiving element (5; 55; 104) that receives each of the optical signals that have passed through the slit pattern (54a; 107), wherein each of the detection tracks is configured from a plurality of slits or reflective zones arranged at regular angular intervals; the slit pattern (54a; 107) in the fixed slit plate (4; 54; 105) is configured from slit rows, the number of slit rows corresponding to the number of detection tracks; the slit rows include a plurality of first slit rows (21, 23, 26) and a plurality of second slit rows (22, 24, 25), the first and second slit rows being lined up in the radial direction (y) of the rotating disc (3; 53; 103); the slits (21a, 23a, 26a) of the first slit rows (21, 23, 26) are arranged at an interval in the circumferential direction that is shorter than the interval of the second slit rows (22, 24, 25); and the slit pattern (54a; 107) is formed so that those slit rows (21, 26) that are located at an innermost and at an outermost position in the radial direction (y) of the rotating disc belong to the first slit rows (21, 23, 26), the radial direction (y) being a direction in which the slit rows are lined up, and the slit pattern (54a; 107) being positioned within an effective irradiation region of the detection light (7; 57; 102) in the fixed slit plate (4; 54; 105).