A molecular beam source for use of thin-film accumulation, for enabling to adjust the volume of molecular beam, which is discharged per an hour with using a needle valve, to be constant irrespective of decrease of a thin-film element forming material remaining within a crucible, comprises heaters 32 and 42 for heating the thin-film element forming materials “a” and “b” within crucibles 31 and 41, and valves 33 and 43 for adjusting the volumes to be discharged of molecules of the thin-film element forming materials “a” and “b”, which are generated within the crucibles 31 and 41. And, it further comprises a control means for adjusting openings of the valves 33 and 43 by means of servomotors 36 and 46 through feeding back of information relating to the volumes of molecular beams, which are obtained from film-thickness meters 16 and 26 for detecting the volume of molecular beams discharged towards the film-forming surface, a heating electric power source for supplying an electric power for heating the heaters 32 and 42, and a control means for adjusting an electric power to be supplied to the heating electric power source depending upon the information relating to the volume of molecular sources and information relating to openings of the valves.