Provided is a device for diluting a viscous substance which is advantages in increasing frequency of contact between the viscous substance and a diluent in order to finely fragment the viscous substance, even when the viscous substance has a high viscosity, and efficiently dilute the viscous substance with the diluent. The device comprises a viscous substance supply portion 27 for supplying the viscous substance to the dilution chamber 20, a rotor 3 rotatably provided in the dilution chamber 20 and finely fragmenting the viscous substance supplied to the dilution chamber 20 by rotation to form a number of small fragments 92 of the viscous substance, and a diluent supply portion 28 for supplying a diluent such as water vapor to the dilution chamber 20 so that the diluent is contacted with the small fragments 92 formed by rotation of the rotor 3.