A particle concentration measuring device includes: a measurement region formation part which has a wall (10) of substantially ring-form and through an inner opening of which gas relatively flows orthogonally; a light curtain forming unit (12A, 12B) forming a planar light curtain (FL) in the inner opening: a particle detecting unit (15) receiving scattered light from particles passing through the light curtain (FL) to detect the particles; and a calculating unit (22) calculating particle concentration based on the total number of the particles detected by the particle detecting unit (15) in a volume of an airflow passing through the light curtain (FL) in a unit time.