Abstract: A fluid mixer device comprising a body (110) having an internal chamber (112) into which there open out at least one hot fluid inlet orifice (116), at least one cold fluid inlet orifice (117), and at least one outlet orifice (124) for the mixture of hot and cold fluids, a slide (150) being received in the chamber (112) and defining, in co-operation with the body (110), a hot fluid passage connected to the hot fluid inlet orifice, and a cold fluid passage connected to the cold fluid inlet orifice, and a thermal expansion element (130) fixed to the slide (150) and suitable for displacing the slide (150) within the chamber (112) in order to modify the ratio of the free fluid flow sections in order to maintain the outlet mixture at a predetermined temperature.