An optical scanning device (1, 101, 161) comprising: a light source which emits light (La); a light deflector (3, 103) having a mirror unit (30, 130) which includes: on a front surface side, a flat reflection part (38, 138) that emits incident light entering from the light source as scanning reflection light (Lb), and a first actuator (32a-32d, 145a,145b) that rotates the mirror unit (30, 130) in a reciprocating manner about a first rotation axis (36, 136); wherein the mirror unit (30, 130) further includes, on the front surface side, a groove-shaped reflection part (39, 40, 69, 139) having a longitudinal groove (41, 71) that extends in parallel to the first axis (36, 136) and that has a pair of inclined surfaces (42a,42b,72a,72b) facing each other such that a groove width decreases from the front surface side to a back surface side, and which reflects the incident light a total of twice, once on each inclined surface and emits the reflected incident light as index light (Lc), and the optical scanning device (1, 101, 161) further comprises: a light detection unit (4a, 4b) which is disposed on a scanning trajectory of a scanning light spot of the scanning reflection light (Lb) and at a light reception position of an index light spot (51a, 51b) of the index light (Lc), and is divided into a first light detector and a second light detector by a division line that divides the index light spot (51a, 51b) in a scanning direction of the scanning reflection light (Lb); and a deflection angle detection unit which detects a first deflection angle as a deflection angle of the mirror unit (30, 130) about the first rotation axis (36, 136) based on comparison between an output of the first light detector and an output of the second light detector. a light deflector (3, 103) having a mirror unit (30, 130) which includes: on a front surface side, a flat reflection part (38, 138) that emits incident light entering from the light source as scanning reflection light (Lb), and a first actuator (32a-32d, 145a,145b) that rotates the mirror unit (30, 130) in a reciprocating manner about a first rotation axis (36, 136); wherein the mirror unit (30, 130) further includes, on the front surface side, a groove-shaped reflection part (39, 40, 69, 139) having a longitudinal groove (41, 71) that extends in parallel to the first axis (36, 136) and that has a pair of inclined surfaces (42a,42b,72a,72b) facing each other such that a groove width decreases from the front surface side to a back surface side, and which reflects the incident light a total of twice, once on each inclined surface and emits the reflected incident light as index light (Lc), and a light detection unit (4a, 4b) which is disposed on a scanning trajectory of a scanning light spot of the scanning reflection light (Lb) and at a light reception position of an index light spot (51a, 51b) of the index light (Lc), and is divided into a first light detector and a second light detector by a division line that divides the index light spot (51a, 51b) in a scanning direction of the scanning reflection light (Lb); and a deflection angle detection unit which detects a first deflection angle as a deflection angle of the mirror unit (30, 130) about the first rotation axis (36, 136) based on comparison between an output of the first light detector and an output of the second light detector.