A gas venting device serves to automatically closing and opening vent holes of a foam mold so that air and reaction product gases are sufficiently exhausted and a foamed product is effectively prevented from extending through the vent holes. The gas venting device includes valves for closing and opening the vent holes, valve driving cylinders for driving the valves, a pilot operated two-position change-over valve, pressure tanks connected through the pilot operated two-position change-over valve to the valve driving cylinders, and mechanical change-over valves connected to the pilot operated two-position change-over valve for changing over it by dogs provided on a passage of the foam mold. The device further includes an air timer for delaying closing the vent holes by the valves.