A fuel vapor processing apparatus includes a canister, a purge passage, a detection passage including an atmosphere passage and a restrictor, a passage changing valve for changing the connection passage of the detection passage between the purge passage and the atmosphere passage, a pump connecting with the detection passage on the opposite side of the passage changing valve across the restrictor, a pressure sensor for detecting a pressure between the restrictor and the pump, and an ECU for computing the concentration of fuel vapor. When the passage changing valve causes the purge passage to connect with the detection passage and the pump reduces pressure in the detection passage to pass the air-fuel mixture through the restrictor, the pressure sensor detects the pressure during a detection period of time that elapses after the air-fuel mixture passes through the restrictor until the air-fuel mixture reaches the pump.