To provide a failure diagnosis for an in-tank canister system having a simple structure, short measurement time, and high reliability; and a method thereof.

The failure diagnosis device for the in-tank canister system includes an atmospheric air shut-off means 16 capable of shutting off an atmospheric air introduction passage 20, a positive pressure generating means 14 applying positive pressure and negative pressure in a canister 10, an evaporated fuel introduction passage opening/closing means 23 opening and closing an evaporated fuel introduction passage according to positive pressure and negative pressure in the canister 10, and a pressure detection means 15 detecting pressure in the canister. The failure diagnosis device for the in-tank canister system is provided with a failure diagnosis means of a canister and the fuel tank diagnosing occurrence of a failure in the canister 10 and the fuel tank 1 when the evaporated fuel introduction passage is opened by pressure measured by a pressure detection means, and it is determined if a failure occurs on the canister when the evaporated fuel introduction passage is closed.