A heat accumulator for vehicle use, attached to a vehicle, has a double tank structure composed of an outer tank and an inner tank, a space formed between the outer tank and the inner tank being maintained as a vacuum, the outer tank and the inner tank each have an opening on a bottom face, the opening of the outer tank and that of the inner tank are joined to each other at the entire peripheries of the openings, preventing means for preventing fatigue failure from starting at the joining portion being provided at a portion extending from the opening to at least a side of the outer tank.