PURPOSE: To form an alloy vapor deposited film of uniform compsn. by installing a thermocouple in a boat for vapor deposition, detecting the arrival of the level of evaporating liquid at a constant level and ending the vapor deposition.

CONSTITUTION: A boat 1 for vapor deposition consisting of W has a groove 6 corresponding to the volume to be left therein out of vapor depositing materials, and is heated by a heater 3 wound with a W coil on a quartz bar in a boat 2 made of quartz glass. The boat 2 has a susceptor 4 made of alumina and a "Chromel-Alumel " thermocouple 5 is fixed to the inside wall near the bottom side of the boat 1. In the stage of charging a vapor depositing material of an alloy in the boat 1 and heating the same to evaporate with the heater 3, the vapor depositing material absorbs the heat of evaporation, and the thermocouple 5 indicates a constant temp. When the evaporating surface lowers below the position of the thermocouple 5, it is detected from the abrupt rise in the surface in the surface temp., thence the evaporation is ceased by leaving the residual component and the segregation of the alloy components near the end of the vapor deposition is prevented.