A fuel vapor purge system comprising: a fuel tank storing fuel; a canister adsorbing vapor fuel emitted from the fuel tank, the canister being able to desorb the vapor fuel; a passage component that defines an intake passage of an internal-combustion engine in which the vapor fuel desorbed from the canister and fuel for combustion are mixed and combusted; a purge pump pumping the vapor fuel from the canister toward the intake passage through a purge passage; a valve device (15, 115, 215, 315) having a main part in which an internal passage is defined and a valve object switching the internal passage between a permission state in which the vapor fuel is allowed to flow into the intake passage and a prohibition state in which the vapor fuel is prevented from flowing into the intake passage, the valve device (15, 115, 215, 315) being disposed to the passage component to control a flow of the vapor fuel pumped by the purge pump; and a control device which controls the valve object between the permission state and the prohibition state, wherein the valve device (15, 115, 215, 315) has an inflow port through which the vapor fuel pumped from the canister flows into the main part, an outflow port connected with the inflow port through the internal passage and being opened to the intake passage, and a leak port (4, 104, 204, 304) connected with the inflow port through the internal passage and being opened to outside of the main part, and the leak port (4, 104, 204, 304) has a leak preventive structure (40, 221; 1040, 1222, 1550, 1221; 2040, 223; 3040, 224) which prevents the vapor fuel from leaking to outside when the valve device (15, 115, 215, 315) is attached to the passage component such that the vapor fuel is able to flow into the intake passage, wherein the leak preventive structure (40, 221; 1040, 1222, 1550, 1221; 2040, 223; 3040, 224) of the leak port (104, 204, 304) comprises a first seal component (40, 1040, 2040, 3040) positioned closer to the main part than a second seal component which seals a clearance between the outflow port and the passage component and wherein the first seal component (40, 1040, 2040, 3040) of the leak preventive structure of the leak port (4, 104, 204, 304) is configured to fail to work prior to the second seal component which seals the clearance between the outflow port and the passage component when the valve device (15, 115, 215, 315) is detached from the passage component. a fuel tank storing fuel; a canister adsorbing vapor fuel emitted from the fuel tank, the canister being able to desorb the vapor fuel; a passage component that defines an intake passage of an internal-combustion engine in which the vapor fuel desorbed from the canister and fuel for combustion are mixed and combusted; a purge pump pumping the vapor fuel from the canister toward the intake passage through a purge passage; a valve device (15, 115, 215, 315) having a main part in which an internal passage is defined and a valve object switching the internal passage between a permission state in which the vapor fuel is allowed to flow into the intake passage and a prohibition state in which the vapor fuel is prevented from flowing into the intake passage, the valve device (15, 115, 215, 315) being disposed to the passage component to control a flow of the vapor fuel pumped by the purge pump; and a control device which controls the valve object between the permission state and the prohibition state, wherein the valve device (15, 115, 215, 315) has an inflow port through which the vapor fuel pumped from the canister flows into the main part, an outflow port connected with the inflow port through the internal passage and being opened to the intake passage, and a leak port (4, 104, 204, 304) connected with the inflow port through the internal passage and being opened to outside of the main part, and an inflow port through which the vapor fuel pumped from the canister flows into the main part, an outflow port connected with the inflow port through the internal passage and being opened to the intake passage, and a leak port (4, 104, 204, 304) connected with the inflow port through the internal passage and being opened to outside of the main part, and the leak port (4, 104, 204, 304) has a leak preventive structure (40, 221; 1040, 1222, 1550, 1221; 2040, 223; 3040, 224) which prevents the vapor fuel from leaking to outside when the valve device (15, 115, 215, 315) is attached to the passage component such that the vapor fuel is able to flow into the intake passage, wherein the leak preventive structure (40, 221; 1040, 1222, 1550, 1221; 2040, 223; 3040, 224) of the leak port (104, 204, 304) comprises a first seal component (40, 1040, 2040, 3040) positioned closer to the main part than a second seal component which seals a clearance between the outflow port and the passage component and wherein the first seal component (40, 1040, 2040, 3040) of the leak preventive structure of the leak port (4, 104, 204, 304) is configured to fail to work prior to the second seal component which seals the clearance between the outflow port and the passage component when the valve device (15, 115, 215, 315) is detached from the passage component.