Hydrogen evolution behavior of an Al-Mg-Si alloy affected by hydrogen embrittlement was investigated using a tensile testing machine equipped with quadruple mass spectrometer in an ultrahigh vacuum chamber. Plate type test pieces were solutionized at 540°C for 1h, quenched in water and then aged at 175°C for 8h or 240h. Some of the aged test pieces were pre-deformed in air with a relative humidity of 90% at a slow strain rate of 8.3x10-7s-1 to introduce hydrogen from the testing atmosphere. As a result of the tensile test with mass spectrometry, it was shown that hydrogen was highly evolved at the moment of fracture. The area fraction of intergranular fracture decreased when the aging condition was changed from peak-aged to over-aged, which was in agreement with the decrease in the amount of hydrogen evolved at the moment of fracture.