A solar light collecting device includes: a rectangular-shaped frame-like body 22 supported by a strut body 21 so as to be tiltable in a vertical plane; a flexible reflecting mirror 24 having two side edges 22a held by the frame-like body 22 and a quasi-parabolic cross section along the short sides thereof to reflect solar light; and a tilting cylinder device 25 which tilts the frame-like body 22 according to the altitude of the sun and guides the light reflected by the reflecting mirror 24 to a heat collecting device disposed at the focal point of the reflecting surface of the reflecting mirror. A curve adjusting device 27 contacts the rear surface of the reflecting mirror 24 according to the tilting position of the reflecting mirror 24 to change the degree of curve of the reflecting surface.