Since the photocharge generation efficiency has reached nearly 100% for recent organic photovoltaic film materials, clarification of loss mechanisms becomes of increasing importance. Here, we report a new time-resolved measurement method that enables simultaneous detection of the optical absorption and electric current due to photogenerated carriers in organic semiconductor films. Microsecond-order decays of both the number density (recombination) and the averaged drift mobility (trapping) of photocarriers are quantitatively observed for an identical device of a regioregular poly[3-hexylthiophene-2,5-diyl]:(6,6)-phenyl C-61 butyric acid methyl ester bulk heterojunction film. The decay feature of the time-dependent mobility is highly dispersive especially at lower temperatures. It can be explained by fast thermalization of "hot" photocarriers into lower energy sites, which is followed by thermally activated hopping. This quantitative time-resolved observation method can be a powerful tool for characterization and optimization of photovoltaic materials.