To provide a leak detecting sensor capable of favorably detecting a leak of a chemical liquid regardless of orientation in fixing to a patient. The leak detecting sensor has a plurality of light-emitting elements 11 each emitting light to be applied to the patient, and a single light-receiving element receiving the light emitted by the plurality of light-emitting elements and reflected by the patient. The plurality of light-emitting elements are placed to surround the single light-receiving element.