Models that a protostar is driving a jet and the jet transfers its momentum to the ambient material and forms a molecular outflow are called as entrainment models. From hydrodynamical calculations, De Young (1986) pointed out there are two mechanisms of entrainment. The ejected jet forms a bow shock and matter hit by the bow shock is accelerated promptly (prompt entrainment). His numerical simulations predicted that gas within the radius from the jet axis of jet radius is entrained in this mechanism. After the bow shock has passed away, slower mechanism of entrainment works, in which a turbulent layer is developed around the jet and the Reynolds stress in the layer transfers the momentum (Stahler 1994). In this mechanism, the ratio of entrained ambient gas to the injected mass by the jet is estimated as (De Young 1986).