Abstract: A laser beam emitted from a laser element is received by a laser receiver. The soot deposition process is carried out by controlling a growing shoot deposition surface at a constant specified position based upon information carried by the received laser beam. The laser receiver is provided with a filter for absorbing heat rays generated by a burner flame and a heat shielding plate made of material having a high heat resistance to prevent the temperature of the receiver from rising. The receiver is also cooled with a stream of cooling gas. The above-described arrangement can prevent the occurrence of malfunction of the sensor portion due to its temperature rise and assures the reliable position control of the soot deposition, achieving a high accuracy of the soot perform produced.