Handheld Gas detector can be used in hazardous areas indoors and outdoors, where combustible or poisonous gas is produced, stored, used, etc., such as chemical, petroleum, metallurgy, oil depot, LPG (Liquefied Petroleum Gas) station, pharmaceutical, gas transmission and distribution, etc. to detect the leakage and show the leaking concentration with a digital display. The detector can raise the alarming when the detected concentration reaches the preset value.