There is provided a humidification system for a breathing circuit that can detect abnormality in flow of intake air inside an inspiration tube. A humidification section is arranged inside an inspiration tube of a breathing circuit. Further, the humidification system is constructed capable of supplying moisture into intake air flowing inside the inspiration tube by using, for example, hydrophobic hollow fibers or a hydrophobic membrane. The flow section comprises a supply section for supplying moisture to the humidification section and a delivery section for delivering moisture that has been supplied to the humidification section. The control section detects intake air abnormality using difference information for water temperature of the supply section and water temperature of the delivery section.