A chemical detecting apparatus comprises a substrate 10 for a chemical in a gas-to-be-monitored to be adsorbed a substrate adsorption rate improving means 12 which enhances the adsorption of the chemical in the gas-to-be-monitored to the substrate, an infrared light source 20 which applies an infrared light to the substrate with the chemical adsorbed to, and infrared light detector 22 which detects the infrared light which has made multiple reflections in the substrate 10 and exited the substrate. Thus, chemicals present in environments can be detected at high speed and with high sensitivity.