An optical system for a particle analyzer, comprising: an irradiating optical system for irradiating light from a light source to a flow channel in a flow cell through which a sample containing particles flows; and a light receiving optical system for receiving light from the flow cell, wherein the light receiving optical system comprises: a light focusing lens system which includes a light focusing lens (201, 302) for focusing forward scattered light from the particles flowing in the flow channel; a light receiving member for receiving the forward scattered light which is focused by the light focusing lens system; and a beam stopper for blocking direct light from the light source, wherein the beam stopper is provided in a light path between the light focusing lens system and the light receiving member, characterised in that: the light focusing lens (201, 302) has an aspheric lens surface, in that the light focusing lens system is configured such that a focal distance of the forward scattered light passing through a central area of the aspheric lens surface which includes an optical axis is longer than a focal distance of the forward scattered light passing through a peripheral area of the aspheric lens surface outside the central area, and in that: the aspheric lens surface has a gradually increasing curvature toward an outer side in the peripheral area, and the central area has a smaller curvature than the peripheral area. an irradiating optical system for irradiating light from a light source to a flow channel in a flow cell through which a sample containing particles flows; and a light receiving optical system for receiving light from the flow cell, wherein the light receiving optical system comprises: a light focusing lens system which includes a light focusing lens (201, 302) for focusing forward scattered light from the particles flowing in the flow channel; a light receiving member for receiving the forward scattered light which is focused by the light focusing lens system; and a beam stopper for blocking direct light from the light source, wherein the beam stopper is provided in a light path between the light focusing lens system and the light receiving member, a light focusing lens system which includes a light focusing lens (201, 302) for focusing forward scattered light from the particles flowing in the flow channel; a light receiving member for receiving the forward scattered light which is focused by the light focusing lens system; and a beam stopper for blocking direct light from the light source, wherein the beam stopper is provided in a light path between the light focusing lens system and the light receiving member, characterised in that: the light focusing lens (201, 302) has an aspheric lens surface, in that the light focusing lens system is configured such that a focal distance of the forward scattered light passing through a central area of the aspheric lens surface which includes an optical axis is longer than a focal distance of the forward scattered light passing through a peripheral area of the aspheric lens surface outside the central area, and in that: the aspheric lens surface has a gradually increasing curvature toward an outer side in the peripheral area, and the central area has a smaller curvature than the peripheral area.