In this vacuum heat insulator, an excellent heat insulating performance is obtained even at high temperature, and this excellent heat insulating performance is maintained for a long period. The hot insulating device and electric water heater using this vacuum heat insulator exhibit an excellent hot insulating performance, and are decreased in the power consumption for hot insulation. The vacuum heat insulator includes a laminate bag, and an insulating core placed in the laminate bag, and the inside of the laminate bag is evacuated in a vacuum state. The laminate bag is made of a laminate film. The laminate film includes a support layer, a deposition layer evaporated on the surface of the support layer, a protective layer placed at the surface side of the deposition layer, and a seal layer placed at the back side of the deposition layer. The deposition layer is formed of at least one material of metal and metal oxide. In this laminate film, (i) the support layer has a plastic film having a glass transition point of 87 C. or higher, (ii) the protective layer has a plastic film having a glass transition point of 87 C. or higher, (iii) the deposition layer has a property of transmitting high frequency magnetic field, or (iv) the laminate bag has a seal portion formed by junction of the seal layer, and the laminate film further as a metal foil placed at a position excluding the seal portion.