Provided is a head-mounted display optical system (LS) having: an optical deflection element (M1); a first lens group (G1); a second lens group (G2); and an optical reflection element (M2). The head-mounted display optical system is configured such that light from a light source, which is reflected on a reflection surface (M2r) of the optical reflection element (M2) and reaches a drawing surface (I) assumed to be located on a retina when a user wears the head-mounted display, moves on the drawing surface (I) according to the change of a travelling direction of the light caused by the optical deflection element (M1), and an image is drawn on the drawing surface (I). The first lens group (G1) includes a free-shaped surface lens having a free-shaped surface which is rotationally asymmetrical with respect to a reference axis, and the reflection surface (M2r) of the optical reflection element (M2) is formed to be rotationally asymmetrical with respect to a reference axis.