Abstract: To improve an offset accuracy in a particle number counting apparatus (100) condensing a working liquid with sample particles, the apparatus including: a porous member (22) in which a flow path (23) is passed through to form an inlet (23a) at a lower side and an outlet (23b) at an upper side thereof so that the working liquid is supplied to a predetermined section of the porous member so as to be impregnated into the entire part of the porous member; and a housing (21) having an accommodating space for accommodating the porous member (22), wherein a gap (S) is formed between an outer peripheral surface (22a) of the porous member (22) and an inner peripheral surface (21b) of the accommodating space of the housing (21) at least in an upper side than the predetermined section to be supplied with the working liquid.