Evaporated fuel processing device

An evaporated fuel processing device mounted on a vehicle, the evaporated fuel processing device comprising:

• a canister configured to adsorb fuel evaporated in a fuel tank;
• a purge passage connected between the canister and an intake passage of an engine, and through which a purge gas sent from the canister to the intake passage passes;
• a pump configured to send the purge gas from the canister to the intake passage;
• a control valve disposed on the purge passage and configured to switch between a communication state and a cutoff state, the communication state being a state where the canister and the intake passage communicate through the purge passage, and the cutoff state being a state where the canister and the intake passage are cut off on the purge passage;
• a branch passage branching from the purge passage at an upstream end of the branch passage and joining the purge passage at a downstream end of the branch passage, the downstream end of the branch passage being located at different position from the upstream end of the branch passage;
• a pressure specifying unit comprising a small diameter portion disposed on the branch passage and through which the purge gas in the branch passage passes, and configured to specify a pressure difference of the purge gas passing through the small diameter portion between an upstream side and a downstream side of the small diameter portion;
• an air-fuel ratio sensor disposed on an exhaust passage of the engine;
• and
• an estimation unit configured to estimate a first flow rate of the purge gas sent from the pump by using an evaporated fuel concentration in the purge gas that is estimated using an air-fuel ratio acquired from the air-fuel ratio sensor and the pressure difference specified by the pressure specifying unit.