A projector type headlamp comprising a projection lens unit located along an optical axis that extends in the longitudinal direction of a vehicle and a light source unit located in the rear of the back focal plane of the projection lens unit, the projector type headlamp being configured such that light distribution patterns of the high beam and low beam are formed by the single projection lens unit, wherein the projection lens unit is composed of two synthetic resin lenses (22, 24) located along the optical axis, the light source unit is configured to form an image of the light source (301, 303) on or in the vicinity of the back focal plane of the projection lens unit and to illuminate an area ahead through the two synthetic resin lenses (22, 24) for formation of the light distribution patterns with boundaries between light and darkness, a diffraction grating is provided on at least a part of a lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens located closer to the light source unit of the two synthetic resin lenses (22, 24), a lens surface on the side of the light source unit of the light-source-side synthetic resin lens has a positive power, the diffraction grating is designed so as to cancel chromatic aberrations caused by light from the light source unit illuminating an area ahead through the two synthetic resin lenses (22, 24), and when the point of intersection between the optical axis and a plane perpendicular to the optical axis is defined as the origin point O, an x axis in the horizontal direction and a y axis in the vertical direction are defined in the plane, the maximum value of y coordinate on the lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens is represented as R1, and a constant that is equal to or greater than 0 and smaller than 1 is represented as A, characterized in that an area that satisfies y < A·R1 on the lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens is formed by a continuous curved surface or a flat surface at least partially provided with the diffraction grating and an area (AR2, 245) that satisfies y ≥ A·R1 is formed by another curved surface that has a power greater than a power of the continuous curved surface or the flat surface and that is not provided with a diffraction grating. wherein the projection lens unit is composed of two synthetic resin lenses (22, 24) located along the optical axis, the light source unit is configured to form an image of the light source (301, 303) on or in the vicinity of the back focal plane of the projection lens unit and to illuminate an area ahead through the two synthetic resin lenses (22, 24) for formation of the light distribution patterns with boundaries between light and darkness, a diffraction grating is provided on at least a part of a lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens located closer to the light source unit of the two synthetic resin lenses (22, 24), a lens surface on the side of the light source unit of the light-source-side synthetic resin lens has a positive power, the diffraction grating is designed so as to cancel chromatic aberrations caused by light from the light source unit illuminating an area ahead through the two synthetic resin lenses (22, 24), and when the point of intersection between the optical axis and a plane perpendicular to the optical axis is defined as the origin point O, an x axis in the horizontal direction and a y axis in the vertical direction are defined in the plane, the maximum value of y coordinate on the lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens is represented as R1, and a constant that is equal to or greater than 0 and smaller than 1 is represented as A, characterized in that an area that satisfies y < A·R1 on the lens surface on the opposite side from the light source unit of the light-source-side synthetic resin lens is formed by a continuous curved surface or a flat surface at least partially provided with the diffraction grating and an area (AR2, 245) that satisfies y ≥ A·R1 is formed by another curved surface that has a power greater than a power of the continuous curved surface or the flat surface and that is not provided with a diffraction grating.