The present invention provides a scanning optical system and radar that can suppress longitudinal distortion and spot rotation of a spot light radiated on an object. A light flux emitted from a light source is reflected on a first mirror surface of a mirror unit, then, proceeds to a second mirror surface, further is reflected on the second mirror surface, and is projected so as to scan on an object correspondingly to rotation of the mirror unit. The light flux emitted from the light projecting system is made longer in a sub scanning angle direction than in a scanning angle direction in a measurement range of the object and satisfies the following conditional expression, |⋅1−90|×|⋅|⋅255 . . . (1); in the expression, ⋅1 is an intersection angle (°) between the first mirror surface and the second mirror surface, and ⋅ is a rotation angle (°).