Overfill prevention valve

An overfill prevention valve disposed in an evaporant line of a fuel tank device, the fuel tank device having a full-tank regulation valve which closes an outlet of fuel vapor when a fuel level reaches a predetermined height at refueling time, and the evaporant line through which an inside of the fuel tank communicates with an outside canister, the overfill prevention valve including:

• a valve housing having
• a first opening which communicates with the fuel tank,
• a second opening which communicates with the canister,
• a first valve seat provided in the first opening, and
• a second valve seat provided in the second opening;
• a parent valve which is disposed slidably within the valve housing so as to come close to and go away from the first valve seat and the second valve seat, and which has an internal space within which the parent valve having:
• a base end opening communicating with the first opening,
• a front end opening communicating with the second opening, and
• a third valve seat provided in the base end opening;
• a first urging unit which urges the parent valve toward the first valve seat;
• a child valve which is disposed slidably within the parent valve so as to come close to and go away from the third valve seat;
• a second urging unit which is disposed within the parent valve and which urges the child valve toward the third valve seat;
• and
• an air passage forming unit which forms an air passage between the parent valve and the first valve seat when the parent valve abuts on the first valve seat;
• wherein an urging force of the first urging unit and an urging force of the second urging unit are set such that, in a state where the parent valve slides against the urging force of the first urging unit and abuts on the second valve seat due to an internal pressure of the fuel tank, a valve opening pressure of the child valve against the third valve seat is higher than a valve re-opening pressure of the parent valve against the second valve seat.