A valve for preventing backflow, comprising: a housing that includes a motor mounted therein and in which a supply flow path and an outlet of air are formed; a driver including a gear part for transmitting driving force of the motor and a converter for converting rotation of the gear part to linear motion; and a valve module that is coupled to an end of the converter and is opened while being spaced apart from the outlet of the housing to be connected to the supply flow path or is closed while being in contact with the outlet, wherein the driver includes a stem that is connected to the valve module and moves in a straight line, wherein a support of the valve module is coupled and fixed to the stem, and wherein a closing part of the valve module is provided to relatively move with respect to the support and the stem, is provided to be moved by a pressure of air applied to the valve module, closes the outlet by receiving a pressure of backflow air when backflow applied to the valve module is generated, is lowered according to a larger air pressure than elastic force of an elastic part, applied from an upper part of the closing part, and is raised according to a pressure of backflow air applied from a lower part of the closing part, characterized in that the closing part includes an outer closing part that selectively comes in contact with the edge of the outlet and performs a sealing function, wherein the stem passes through a center of the outer closing part, and wherein a stem sealing member that is in contact with an outer circumference of the stem and performs the sealing function is provided on the outer closing part, and wherein the stem sealing member is configured to support sliding movement of the outer closing part on the outer circumference of the stem. a housing that includes a motor mounted therein and in which a supply flow path and an outlet of air are formed; a driver including a gear part for transmitting driving force of the motor and a converter for converting rotation of the gear part to linear motion; and a valve module that is coupled to an end of the converter and is opened while being spaced apart from the outlet of the housing to be connected to the supply flow path or is closed while being in contact with the outlet, wherein the driver includes a stem that is connected to the valve module and moves in a straight line, wherein a support of the valve module is coupled and fixed to the stem, and wherein a closing part of the valve module is provided to relatively move with respect to the support and the stem, is provided to be moved by a pressure of air applied to the valve module, closes the outlet by receiving a pressure of backflow air when backflow applied to the valve module is generated, is lowered according to a larger air pressure than elastic force of an elastic part, applied from an upper part of the closing part, and is raised according to a pressure of backflow air applied from a lower part of the closing part, characterized in that the closing part includes an outer closing part that selectively comes in contact with the edge of the outlet and performs a sealing function, wherein the stem passes through a center of the outer closing part, and wherein a stem sealing member that is in contact with an outer circumference of the stem and performs the sealing function is provided on the outer closing part, and wherein the stem sealing member is configured to support sliding movement of the outer closing part on the outer circumference of the stem.