The present invention provides a method for manufacturing a closed impeller that has a simple configuration and enables fusion in a very accurate position. The method includes: forming a frustum-shaped front plate and a disk-shaped base plate formed therebelow, upper surfaces of a plurality of unit impeller blade plates made from a synthetic resin extending in radial directions which are formed as impeller blade tip surfaces, with sharp-pointed ridge-like protrusions having an acute upper end and a width less than a width of the impeller blade tip surface being integrally formed on the impeller blade tip surfaces. In the method, an inner surface of the front plate and the plurality of impeller blade tip surfaces are formed as parts of conical surfaces and formed so that the cone apex angles of the two conical surfaces are equal to each other. Then, the front plate is placed on the impeller blade and the ends of the sharp-pointed ridge-like protrusions and the inner surface of the front plate are brought into contact with each other. Small vibrations are then provided while applying an appropriate pressurizing force between the front plate and the impeller blade having the base plate attached thereto, the sharp-pointed ridge-like protrusions are melted, and the front plate is fused to the tip surfaces of the impeller blade.