A diffusion light distribution optical system for a vehicle lighting apparatus, whereby the diffusion light distribution optical system comprises a lens body that diffusively distributes light emitted from a light source toward a vehicle travel direction and that is configured such that a plurality of the lens bodies are arranged to be aligned in a vehicle width direction, wherein: the lens body has a first lens unit that includes a first incidence surface a reflection surface and a first emission surface and a second lens unit that includes a second incidence surface and a second emission surface, the lens body being configured such that light from the light source is incident on the first incidence surface to enter an inside of the first lens unit, part of the light is reflected by the reflection surface, then the light is emitted to an outside of the first lens unit from the first emission surface, the light is further incident on the second incidence surface to enter an inside of the second lens unit, the light is emitted to an outside of the second lens unit from the second emission surface, and thereby, the light emitted frontward of the lens body forms a predetermined light distribution pattern which has an upper end edge including a cutoff line defined by a front end part of the reflection surface; the first emission surface is configured as a lens surface having a semicircular column shape having a cylindrical axis that extends in a vertical direction such that the light emitted from the first emission surface is focused in a horizontal direction; the second emission surface is configured as a lens surface having a semicircular column shape having a cylindrical axis that extends in a horizontal direction such that the light emitted from the second emission surface is focused in a vertical direction; the second emission surfaces of the plurality of lens bodies form a continuous emission surface having a semicircular column shape and extending in a line in the vehicle width direction in a state where the second emission surfaces are adjacent to each other; and one or more lens bodies of the plurality of lens bodies are arranged in a state where an optical axis of the first lens unit is slanted with respect to the vehicle travel direction. the lens body has a first lens unit that includes a first incidence surface a reflection surface and a first emission surface and a second lens unit that includes a second incidence surface and a second emission surface, the lens body being configured such that light from the light source is incident on the first incidence surface to enter an inside of the first lens unit, part of the light is reflected by the reflection surface, then the light is emitted to an outside of the first lens unit from the first emission surface, the light is further incident on the second incidence surface to enter an inside of the second lens unit, the light is emitted to an outside of the second lens unit from the second emission surface, and thereby, the light emitted frontward of the lens body forms a predetermined light distribution pattern which has an upper end edge including a cutoff line defined by a front end part of the reflection surface; the first emission surface is configured as a lens surface having a semicircular column shape having a cylindrical axis that extends in a vertical direction such that the light emitted from the first emission surface is focused in a horizontal direction; the second emission surface is configured as a lens surface having a semicircular column shape having a cylindrical axis that extends in a horizontal direction such that the light emitted from the second emission surface is focused in a vertical direction; the second emission surfaces of the plurality of lens bodies form a continuous emission surface having a semicircular column shape and extending in a line in the vehicle width direction in a state where the second emission surfaces are adjacent to each other; and one or more lens bodies of the plurality of lens bodies are arranged in a state where an optical axis of the first lens unit is slanted with respect to the vehicle travel direction.