A gas detection-use image processing device is provided with a first input unit on which an operation of inputting a flow rate of gas used as an index of a gas concentration level which is wanted to be detected is performed to input the flow rate, a second input unit to which an image of an imaging target taken by the imaging device is input, and a first calculation unit which calculates, when the image is taken in a state in which the gas of the flow rate appears in an imaging range of the imaging device, a region in which the gas may be visualized in the imaging range.