A fluid measurement method for visualising fluid flow, comprising: a preparation process of dissolving in a fluid a photochromic compound having an amount of absorption of light that changes upon irradiation with transformation-inducing light; a transformation-inducing irradiation process of irradiating the fluid with the transformation-inducing light that causes photochromism; a post-transformation imaging process of taking an image of the fluid after irradiation with the transformation-inducing light; and an image processing process that comes after the post-transformation imaging process, characterized in that the post-transformation imaging process includes: irradiating the fluid with illumination light that includes first light and second light, the first light being in a first wavelength range across which an amount of absorption of light changes upon irradiation with the transformation-inducing light and the second light being in a second wavelength range across which an amount of absorption of light does not change or hardly changes; separating the illumination light into the first light and the second light after the illumination light has been transmitted through the fluid; generating a first image by taking an image of the fluid by using the separated first light; and generating a second image by taking an image of the fluid at the same time as the first image is taken but by using the separated second light, and wherein the image processing process generates a third image by using at least the first image and the second image. a preparation process of dissolving in a fluid a photochromic compound having an amount of absorption of light that changes upon irradiation with transformation-inducing light; a transformation-inducing irradiation process of irradiating the fluid with the transformation-inducing light that causes photochromism; a post-transformation imaging process of taking an image of the fluid after irradiation with the transformation-inducing light; and an image processing process that comes after the post-transformation imaging process, characterized in that the post-transformation imaging process includes: irradiating the fluid with illumination light that includes first light and second light, the first light being in a first wavelength range across which an amount of absorption of light changes upon irradiation with the transformation-inducing light and the second light being in a second wavelength range across which an amount of absorption of light does not change or hardly changes; separating the illumination light into the first light and the second light after the illumination light has been transmitted through the fluid; generating a first image by taking an image of the fluid by using the separated first light; and generating a second image by taking an image of the fluid at the same time as the first image is taken but by using the separated second light, and wherein the image processing process generates a third image by using at least the first image and the second image. irradiating the fluid with illumination light that includes first light and second light, the first light being in a first wavelength range across which an amount of absorption of light changes upon irradiation with the transformation-inducing light and the second light being in a second wavelength range across which an amount of absorption of light does not change or hardly changes; separating the illumination light into the first light and the second light after the illumination light has been transmitted through the fluid; generating a first image by taking an image of the fluid by using the separated first light; and generating a second image by taking an image of the fluid at the same time as the first image is taken but by using the separated second light, and wherein the image processing process generates a third image by using at least the first image and the second image.