An image-processing device, for gas detection that performs image processing on infrared images of a subject being taken at plurality of times a day, includes a hardware processor that generates physical-quantity-change data indicating chronological change in physical quantity determined based on pixel data of pixels constituting the infrared image. The hardware processor selects, from the pixels constituting the infrared image, a reference pixel used as a reference and a comparison pixel to be compared with the reference pixel, and calculates a degree of phase similarity indicating a degree of similarity in phase between a waveform of the physical-quantity-change data of the reference pixel and a waveform of the physical-quantity-change data of the comparison pixel. The hardware processor determines, based on the degree of phase similarity, that an image including the reference pixel and the comparison pixel is not a gas image.