A gas detecting device includes a casing, a gas transportation actuator, a gas detector and a driving module. The casing has an inlet, an outlet and an accommodation slot. The inlet and the outlet are in fluid communication with each other, and the accommodation slot is disposed under the inlet and is in fluid communication with the inlet. The gas transportation is disposed within and seals the accommodation slot, and the gas transportation actuator is enabled to introduce air thereinto through the inlet and discharge the air through the outlet. The gas detector is configured to detect an amount of a specific gas in the air introduced through the inlet. The driving module controls the actuations of the gas transportation actuator and the gas detector, so that the gas detector detects the amount of the specific gas in the air.