PURPOSE: To perform reliable clamping and fastening by driving a screw thread into the mounting hole formed with the specified hole diameter in a boss projected with the specified outer diameter in reference to the screw thread of the tapping screw of a product made of synthetic resin while forcing the screw thread to bite and cut a hole wall.

CONSTITUTION: A screw thread 6 is driven into a mounting hole 17 having its hole diameter in a range from 0.65 to 0.90d, which is formed in a boss 16 having its diameter of 1.70 or more in reference to the diameter (d) of the screw thread of the tapping screw 1 of a molded product 15 made of synthetic resin, while being forced to bit and cut a hole wall. The screw thread 6 is driven while being forced to bite and cut the hole wall of the mounting hole 17 and although the synthetic resin pushed out by the screw thread 6 makes pressure contact with the root section 7 of the screw thread 6, because it is received by a conic face 9, the pressure in the radial direction is reduced and driving torque is not increased. In addition, since proper selection of the outer diameter of the boss 16 and the hole diameter of the mounting hole is made, driving can be made by small driving torque and the boss 16 is neither broken nor expanded to much extent.