A gas detection device manufactured by a semiconductor process includes a substrate, a microelectromechanical element, a light-emitting element, a particle-sensing element, a gas-sensing element, a driving-chip element and an encapsulation layer. The driving-chip element controls driving operations of the microelectromechanical element, the light-emitting element, the particle-sensing element and the gas-sensing element, respectively. When the microelectromechanical element is enabled to actuate transportation of gas, the gas is introduced into the gas detection device through an inlet aperture of the substrate. Scattered light spots generated by the light beam of the light-emitting element irradiating on suspended particles contained in the gas are received by the particle-sensing element to generate a detection datum of the suspended particles. The gas-sensing element detects the gas passing through and generates a detection datum of hazardous gas contained in the gas. Finally, the gas is discharged from an outlet aperture of the encapsulation layer.