Provided is a device for diluting a viscous substance which is advantageous in spreading a viscous substance to increase its area and allow efficient dilution of the viscous substance with a diluent. The device comprises a base body having a treatment chamber to be supplied with a viscous substance and a diluent for diluting the viscous substance; a surface element having a coating surface to be attached by the viscous substance supplied to the treatment chamber of the base body; and a movable member movable along the coating surface of the surface element and a coating element. The coating element is provided on the movable member and mechanically spreads the viscous substance attached to the coating surface of the surface element on the coating surface of the surface element in association with movement of the movable member to increase area of the viscous substance.