There are provided a reflection film, a windshield glass, and a head-up display system capable of suppressing formation of double images of a display image. The reflection film has a linearly polarized light reflection layer in which optically anisotropic layers and isotropic layers are laminated and a polarization converting layer, and the polarization converting layer satisfies any of conditions below.
(A) The polarization converting layer is a retardation layer in which the front retardation is 30 nm to 200 nm and the angle between a slow axis direction and a direction of the transmission axis of the linearly polarized light reflection layer is 35° or less.
(B) The polarization converting layer is a layer in which a helical alignment structure of a liquid crystal compound is fixed, and the number of pitches x in the helical alignment structure and the film thickness y (unit μm) of the polarization converting layer satisfy all relational expressions below.

0.1≤x≤1.0  (i)

0.5≤y≤3.0  (ii)

3000≤(1560×y)/x≤50000  (iii)