A light-guiding device that converts an input beam bundle made of a plurality of input beams to an output beam bundle made of a plurality of output beams, comprising: double mirrors (Mi) which (i) correspond to the respective input beams and (ii) are separated from each other, each of the double mirrors (Mi) being made of a first mirror that is mounted on a certain flat surface and a second mirror that is mounted on the first mirror, the first mirror having a first reflective surface which (i) reflects a corresponding one of the input beams and (ii) is arranged so that there is an angle of θ1 between the certain flat surface and the first reflective surface, the second mirror having a second reflective surface which (i) reflects the corresponding one of the input beams which has been reflected by the first reflective surface and (ii) is arranged so that there is an angle of θ2 between the certain flat surface and the second reflective surface, the θ1 satisfying the following expressions: 90 ° − θ angle = − θ 2 y × 2 − 90 ° − φ 1 × 2 − 90 ° − θ 1 × 2; and θ 1 ≠ 45 °, and the θ2 satisfying the following expressions: 90 ° − φ 2 = − θ 1 y × 2 − 90 ° − θ 2 × 2; and θ 2 ≠ 45 °, where (i) θ angle is an angle between the corresponding one of the input beams and a corresponding one of the output beams, (ii) φ 1 is an angle between an optical axis of the corresponding one of the input beams and a normal of the certain flat surface, (iii) φ 2 is an angle between an optical axis of the corresponding one of the output beams and the normal of the certain flat surface, (iv) θ1y is a rotation angle which shows how much the first mirror has rotated from a first reference direction around the normal of the certain flat surface serving as a rotation axis, which first reference direction is a direction of the first mirror while an orthogonal projection of a normal vector of the first reflective surface onto the certain flat surface is parallel to an orthogonal projection of the optical axis of the corresponding one of the input beams onto the certain flat surface, and (v) θ 2y is a rotation angle which shows how much the second mirror has rotated from a second reference direction around the normal of the certain flat surface serving as a rotation axis, which second reference direction is a direction of the second mirror while an orthogonal projection of a normal vector of the second reflective surface onto the certain flat surface is orthogonal to the orthogonal projection of the optical axis of the corresponding one of the input beams onto the certain flat surface. double mirrors (Mi) which (i) correspond to the respective input beams and (ii) are separated from each other, each of the double mirrors (Mi) being made of a first mirror that is mounted on a certain flat surface and a second mirror that is mounted on the first mirror, the first mirror having a first reflective surface which (i) reflects a corresponding one of the input beams and (ii) is arranged so that there is an angle of θ1 between the certain flat surface and the first reflective surface, the second mirror having a second reflective surface which (i) reflects the corresponding one of the input beams which has been reflected by the first reflective surface and (ii) is arranged so that there is an angle of θ2 between the certain flat surface and the second reflective surface, the θ1 satisfying the following expressions: 90 ° − θ angle = − θ 2 y × 2 − 90 ° − φ 1 × 2 − 90 ° − θ 1 × 2; and θ 1 ≠ 45 °, and the θ2 satisfying the following expressions: 90 ° − φ 2 = − θ 1 y × 2 − 90 ° − θ 2 × 2; and θ 2 ≠ 45 °, where (i) θ angle is an angle between the corresponding one of the input beams and a corresponding one of the output beams, (ii) φ 1 is an angle between an optical axis of the corresponding one of the input beams and a normal of the certain flat surface, (iii) φ 2 is an angle between an optical axis of the corresponding one of the output beams and the normal of the certain flat surface, (iv) θ1y is a rotation angle which shows how much the first mirror has rotated from a first reference direction around the normal of the certain flat surface serving as a rotation axis, which first reference direction is a direction of the first mirror while an orthogonal projection of a normal vector of the first reflective surface onto the certain flat surface is parallel to an orthogonal projection of the optical axis of the corresponding one of the input beams onto the certain flat surface, and (v) θ 2y is a rotation angle which shows how much the second mirror has rotated from a second reference direction around the normal of the certain flat surface serving as a rotation axis, which second reference direction is a direction of the second mirror while an orthogonal projection of a normal vector of the second reflective surface onto the certain flat surface is orthogonal to the orthogonal projection of the optical axis of the corresponding one of the input beams onto the certain flat surface.