A gas detection system, comprising: a humidity measuring apparatus that measures humidity; a transmitting apparatus that includes a light source which emits an optical signal of a plurality of wavelengths; and a receiving apparatus including: a light detection part that receives the optical signal from the transmitting apparatus; a discrimination part that determines whether or not condensation exists in at least one region of regions where the optical signal passes, based on the humidity measured by the humidity measuring apparatus and a light intensity of the optical signal received by the light detection part.