A gas sensing device is adapted to detect a concentration of a target gas. The gas sensing device comprises a dielectric layer, a reference sensing portion, a target sensing portion, and a controller. The dielectric layer is disposed on a surface. The reference sensing portion and the target sensing portion are respectively disposed on a supporting side of the dielectric layer, with said supporting side facing away from the surface. The reference sensing portion includes a first conductive layer and a first sensing layer with low sensitivity to the target gas. The target sensing portion includes a second conductive layer and a second sensing layer with high sensitivity to the target gas. The controller obtains the immittance values of the first conductive layer and the second conductive layer, and calculates a concentration value of the target gas according to the immittance values and a correlation function.