PROBLEM TO BE SOLVED: To solve a problem that CO emission increases as an ignition frequency increases in a carbon dioxide generator in which a secondary combustion plate does not have heat enough to burn unburned gas again at the time of ignition and thus a small quantity of CO is generated, in management and application of carbon dioxide concentration with a CO2 concentration sensor, that is a method of shortening combustion time and increasing a combustion frequency, to keep CO2 at a high concentration in the daytime when photosynthesis is active.SOLUTION: There is provided an outlet mechanism of a carbon dioxide generator in which an outlet of the carbon dioxide generator has a structure capable of opening and closing in the inner and outer direction in a greenhouse, the opening and closing are controlled with an opening and closing valve arranged in the outlet, and the opening and closing valve has a valve constitution of an operation mechanism and drive means, a valve constitution of a shape-memory alloy and an actuator, or a valve constitution of manual means. It is useful to eliminate harmful effects of CO even if the ignition and halt of the carbon dioxide generator are appropriately switched.