A vacuum thermally bonding apparatus is provided, in which while air is being prevented in vacuum from entering a bonding layer, an element is thermally bonded to a substrate under vacuum by forming the bonding layer having a good thickness under an appropriate pressing pressure, while suppressing runout of the adhesive as much as possible, with a pressing force being slightly adjustable. A lower end portion of an upper frame member is gas-tightly slidably sealed to a peripheral portion of the lower plate member to form a vacuum partition wall therein, and a pressurizing release film is contacted with an upper face of the element, and thermally softened in the atmospheric pressure. A vacuum chamber is evacuated to vacuum, and the lower plate member and an intermediate member are relatively moved in an approaching direction, so that an outer peripheral portion of the pressurizing release film is gas-tightly held between the upper face of the substrate-placing table of the lower plate member and a lower face of an inner frame body. By applying the atmospheric pressure or a pressure higher than the atmospheric pressure to a space above the pressurizing release film inside the vacuum chamber, the pressurizing release film is adhered onto the outer surfaces of the substrate and the element, and the element is bonded to the substrate.