We report on a three-color InAs/GaAs quantum dot infrared photodetector grown by molecular beam epitaxy. The InAs quantum dots with AlGaAs inserting layers are formed on an InGaAs quantum well layer as an absorber region. The detector is an nin-type device, and three photocurret peaks at 6.3, 10.2 and 11 m are observed at 77 K, respectively. Each peak is designated and the physical mechanism is discussed. The dependences of the intensities of the three peaks on the applied bias voltage are different for the positive and the negative bias conditions due to the asymmetric structure of the absorber region. The peak arising from the transition between the quantum dot ground state and a continuum state becomes weaker when the bias voltage is larger than a certain value. The physical reason is attributed to the decrease of the wavefunction overlap between the two quantum states.