The carrier mobility of titanyl-phthalocyanine (TiOPc) films was evaluated based on field-effect measurements at the presence of electron acceptor gas (NO2) and electron donor gas (NH3) Molecules, The increase in carrier density due to NO2 adsorption was found to be a dominant factor of an increase in the dark electric conductivity. On the other hand, the decrease in conductivity at the presence of NH3 gas was contributed by the decrease in both carrier mobility and carrier density. The adsorbed gas molecules were found to play an important role not only in carrier density but also in the carrier transport.