PROBLEM TO BE SOLVED: To realize determination of photosynthesis which has been difficult by a simple device, by which supply of carbon dioxide is appropriately controlled.SOLUTION: There is provided a carbon dioxide supply device in cultivation facility where a porous pipe 5 connected to carbon dioxide supply means, such as a carbon dioxide cylinder 1, is arranged in the vicinity of plants A, and a carbon dioxide supply amount from the carbon dioxide supply means is controlled by valve open/close control means (gas supply amount control means) 11. A leakage gas monitoring sensor 8 which detects carbon dioxide concentration in a plant sparse region remote from the porous pipe 5 is arranged, and when a sensor output from the sensor 8 exceeds a prescribed value, the valve open/close control means 11 narrows a carbon dioxide supply amount from the carbon dioxide supply means 1.
