We developed an excretion monitoring system equipped with non-contact temperature sensor and a semiconductor gas sensor installed under a toilet seat. The gas sensor is able to detect odors due to human excretion by transiently increasing the output. The peak amplitude and duration of the gas sensor output due to defecation were significantly greater than those due to urination. These results demonstrate the capability of the gas sensor in distinguishing whether the excretion is urination or defecation.