PURPOSE: To make the heat electromotive force characteristic of a welded thermocoupling uniform regardless of the skill of an operator.

CONSTITUTION: This unit is provided with a fixed electrode 2 having a horizontal electrode surface placed on the tip ends of a pair of crossed thermocouple wires, a movable electrode 3 which pressurizes the thermocouple wires against the fixed electrode 2 with the dead load and is movable up and down, a welding capacitor C2 which supplies an electric current to the thermocouple wires being in the contact state between the fixed electrode 2 and the movable electrode 3, a power charging circuit D1 charging the welding capacitor C2, and a welding voltage setting switch S2 which sets the welding voltage of at the time of the completion of the power charge of the welding capacitor C2 to the required value by selecting one among plural constant-voltage diodes ZD.