To prevent invasion of cement paste into a groove by forming a sealing material mounting groove of an inner surface of a form in a sectional size represented by a specific formula in response to a size of the material, thereby easily incorporating the material in the groove.

A width D of a groove bottom is substantially equal to a width (a) of a sealing material, and D=(0.95 to 1.10)a. A depth (h) of the groove is substantially equal to a thickness (b) of the material, and h=(1.0 to 1.1)b. In addition, an inlet size H of a groove section is formed so that h/H is larger than 0.5 and smaller than 1.5. if the h/H is 0.5 or less, the material 1 is easily removed from the groove 3. Meanwhile, if the h/H is 1.5 or more, the groove 3 is excessively deep, and hence it cannot be snugly mounted in the groove, and hence irregular in shape. And, if (D-H) is less than 1.5mm, an interference is small, and hence the material is easily removed from the groove 3. Meanwhile, if it is larger than 8mm, the material is scarcely mounted in the groove 3. If it is 1.5mm to less than 8mm, its operability is improved, and a cleaning time labor hour of lime wash is almost eliminated.