A particle sensor includes: a detecting area into which a gas including particles is introduced; a light-projecting element; a light-receiving element which receives scattered light of the light reflected by the particles in the gas in the detecting area; a heater which heats the gas; a reflector which directs the scattered light to the light-receiving element, wherein the reflector includes: a first ellipsoidal portion having an inner surface shape that defines a portion of a surface of revolution of a spheroid; and a first spherical portion having an inner surface shape that defines a portion of a spherical surface of a sphere, wherein the first ellipsoidal portion has one focus of the spheroid located in the detecting area and the other focus of the spheroid located at or proximate to the light-receiving element, and the first spherical portion has a center of the sphere located in the detecting area.