Abstract: Disclosed is a diffuser controller for tidal stream power generation, for use in a tidal stream power generator installed under water to generate electricity using a tidal stream. The diffuser controller includes: a turbine configured to be rotated by the tidal stream; a diffuser which is formed with an inlet through which a fluid flows into the diffuser and an outlet through which the fluid flows out from the diffuser, the turbine being fixed inside the diffuser and a rear foil being rotatably installed in the outlet; and a control unit configured to adjust an rotating angle of the rear foil depending on a direction or velocity of the tidal stream. The diffuser is configured to be rotatable.