PURPOSE: To provide a vacuum thermal insulating material having a quite low thermal conductivity and a superior aging deterioration stability characteristic by a method wherein inorganic acid having a specified dissociation constant is applied as a hardening agent to form an acid hardening type continuous air bubble resole form and the resole form is applied as a core member, the core member is stored in a container formed by a synthetic resin film having no aeration characteristics.

CONSTITUTION: An acid hardening type continuous bubble resole form made by applying inorganic acid with a vapor pressure at 100°C being less than 10 mm Hg or a vapor pressure at 25°C being more than 760 mmHg and with its dissociation constant pka being less than 2.5 as a hardening agent is formed as a core member 1. This core member is stored in a container 3 formed by a film 2 having no aeration characteristic such as aluminum vapor deposited or aluminum foil-plastic film and then an interior of the container is evacuated to form vacuum therein, thereafter an opening part of the container is sealingly closed at 31. As inorganic acid, for example, boron trifluoride, hydrogen bro mide, hydrogen chloride or the like can be applied and in particular hydrogen chloride is preferable in these substances.