In order to provide a CO2 diffuser with which it is possible to inexpensively and easily apply CO2 without requiring the use of new piping, the disclosed CO2 diffuser has a structure wherein a watering pipe (3) is connected through a three-way valve (V) to the downstream ends of a CO2 supply path (1) and a water supply path (2). CO2 is applied by diffusing CO2 from the watering pipe (3), and watering is conducted by spraying water from the watering pipe (3) . It is preferred that switching therebetween should be conducted automatically in accordance with data sent from a CO2 sensor. Furthermore, it is preferred that the watering pipe (3) should be a commercial porous pipe, and this porous pipe should be supported above the ground and disposed in the vicinity of the stomata present in the backs of leaves of a plant grown in cultivation facilities.
