Large-diameter projection portions and small-diameter projection portions are formed in each of the two annular plates, while large-diameter end turn portions, which are respectively associated with large-diameter projection portions, and small-diameter end turn portions, which are respectively associated with small-diameter projection portions, are formed in the compression coil springs. Then, a punch having a diameter portion, whose diameter is larger than the inside diameter of the large-diameter projection portion, is inserted thereinto and increases the diameter thereof. Consequently, the large-diameter end turn portions of the compression coil springs are assembled to each of the annular plates. Subsequently, the punch having a portion, whose diameter is larger than the inside diameter of the small-diameter end turn portion is inserted from the inside of each of the enlarged large-diameter projection portions into the small-diameter projection portion and increases the diameter of the small-diameter projection portion.