A head-mounted display optical system includes: a light deflecting element; a first lens group that has a positive refracting power as a whole, includes a plurality of rotationally symmetrical lenses and one free-form surface lens, and condenses the light incident through the light deflecting element; a second lens group that includes one lens having a positive refracting power that is arranged in a vicinity of an imaging position of an intermediate image formed by the first lens group; a third lens group that includes one rotationally symmetrical lens and one free-form surface lens; and a light reflecting element that has a free-form surface shape formed in a rotationally asymmetrical manner with respect to the optical axis and has a reflection surface that reflects the light having transmitted through the third lens group, in a sequence from a side of the light source side.