To provide an outflow prevention device which is capable of preventing refrigerant from flowing out into a vehicle compartment even when an evaporator has ruptured. The outflow prevention device comprises a shut valve arranged in a passage via which refrigerant introduced into a shut valve inlet from an expansion device is allowed to flow to the evaporator, and a check valve arranged in a passage via which refrigerant returned from the evaporator is allowed to flow to an accumulator. A chamber behind a piston fixed to a plug holding a valve sheet and a chamber on a downstream side of the check valve are communicated with each other via a bypass passage, so that differential pressure generated across the check valve by flow of refrigerant through the same can be applied to the shut valve to open and close the same. The valve sheet and the piston are formed to have the same diameters so as to prevent the shut valve from being influenced by introduced pressure, thereby enabling the shut valve to be operated only by the differential pressure generated across the check valve.