PURPOSE: A built-up constant-temperature oven is provided to be easily applied as a high-capacity device by increasing an air volume which flows in and to increase humidifying efficiency and prevent the increase of constant pressure. CONSTITUTION: A built-up constant-temperature oven comprises a constant-temperature oven room (100), a filter unit (200), a humidity control unit (300), a cooling unit (400), a blowing unit (500), and an opening and closing unit (600). The constant-temperature oven room is located within a building, so that an operator is able to have access inside. The filter unit filters the outside air and the inside air which flow in. The humidity control unit controls the humidity of the outside air and the inside air which pass through the filter unit. The cooling unit cools the outside air and the inside air which pass through the filter unit. The blowing unit forcibly provides the outside air and the inside air which flow in to the inside. The opening and closing unit selectively controls the inflow of the outside and inside air.