A particle detector that includes a flow cell that includes a through hole having a circular sectional shape and allowing a fluid containing a particle to flow therethrough, an inspection light source that irradiates the flow cell with inspection light in a direction perpendicular to an extending direction of the through hole, and an optical detector that detects reaction light generated by the particle and which exits the flow cell so as to be angled relative to a sector-shaped plane which has a vertex at an intersection point of the inspection light and the through hole of the flow cell, is parallel to an optical path of the inspection light, and is perpendicular to the extending direction of the through hole of the flow cell.