A head-mounted display optical system, comprising: an optical deflection element that is configured to change a travelling direction of light from a light source; a first lens group that has positive refractive power and that is configured to collect light entered via the optical deflection element; a second lens group that is disposed near a position of intermediate image formed by the first lens group; and an optical reflection element that has a reflection surface (M2r) to reflect light transmitted through the second lens group, and that is configured to allow light, entered from an opposite surface (M2s) to the reflection surface (M2r), to transmit therethrough, the head-mounted display optical system being configured such that light from the light source, which is reflected on the reflection surface (M2r) and reaches a drawing surface assumed to be located on a retina when a user wears the head-mounted display, moves on the drawing surface in accordance with the change of a travelling direction of the light caused by the optical deflection element, and an image is drawn on the drawing surface, the first lens group including a free-shaped surface lens (L11, L12, L13) having a free-shaped surface which is rotationally asymmetrical with respect to a first reference axis, the reflection surface (M2r) of the optical reflection element being formed to be rotationally asymmetrical with respect to a second reference axis, and the following conditional expressions being satisfied: 0.20 < m < 3.00 20 < f 2 / f 1 < 3000 where m denotes an afocal magnification of the head-mounted display optical system, f1 denotes a focal length of the first lens group, and f2 denotes a focal length of the second lens group. an optical deflection element that is configured to change a travelling direction of light from a light source; a first lens group that has positive refractive power and that is configured to collect light entered via the optical deflection element; a second lens group that is disposed near a position of intermediate image formed by the first lens group; and an optical reflection element that has a reflection surface (M2r) to reflect light transmitted through the second lens group, and that is configured to allow light, entered from an opposite surface (M2s) to the reflection surface (M2r), to transmit therethrough, the head-mounted display optical system being configured such that light from the light source, which is reflected on the reflection surface (M2r) and reaches a drawing surface assumed to be located on a retina when a user wears the head-mounted display, moves on the drawing surface in accordance with the change of a travelling direction of the light caused by the optical deflection element, and an image is drawn on the drawing surface, the first lens group including a free-shaped surface lens (L11, L12, L13) having a free-shaped surface which is rotationally asymmetrical with respect to a first reference axis, the reflection surface (M2r) of the optical reflection element being formed to be rotationally asymmetrical with respect to a second reference axis, and the following conditional expressions being satisfied: 0.20 < m < 3.00 20 < f 2 / f 1 < 3000 where m denotes an afocal magnification of the head-mounted display optical system, f1 denotes a focal length of the first lens group, and f2 denotes a focal length of the second lens group.