a connection mode between the canister 14 and the atmospheric air introduction passage 30 through the paths a pressure sensor 52 functioning as a detector for detecting a pressure of the paths and an electric pump 38 in these three paths a main path 40 corresponds to a path for directly connecting the canister 14 and the atmospheric air introduction passage 30 and a determination path 42 corresponds to a path connecting the canister 14 and the atmospheric air introduction passage 30 via a pump 38 further a reference path 44 in the three paths corresponds to a path connecting the canister 14 and the atmospheric air introduction passage 30 via the pump 38 in the same manner as the determination path 42 a throttle 46 is provided in the reference path 44 the pump 38 mentioned above forcibly discharges the air within the fuel vapor path 24 the determination path 42 and the reference path 44 to an outside an intake side of the pump 38 is provided with a check valve 48 which is opened at a time when the pressure in the intake side is lowered by the operation of the pump 38 and the pressure sensor 52 further the canister 14 and the atmospheric air introduction passage 30 are connected via the main path 40 at a time when.