Abstract: Two fluids having different temperatures are to be mixed while supplying one fluid (A) through a main pipe (1) and conducting the other fluid (B) from a secondary pipe (2) into the main pipe. A mixing device (6) comprises a connecting branch which extends essentially radially into the main pipe (1) from the secondary pipe (2) and along whose outside the first fluid (A) may pass and which is formed with at least one nozzle-shaped aperture through which the second fluid (B) can be conducted into and mixed with the passing first fluid. The connecting branch comprises at least one through channel (13) extending transversely of the connecting branch and suitably in parallel with the longitudinal extent of the main pipe (1) and through which part of the first fluid can pass in a central partial flow.