To extremely minimize the effect by the self weight of a base in annealing and retain well the mechanical characteristics by heating the resin base after molding inside a liquid inactive liquid to the base resin at a sepecified temperature.

A resin base after injection molding is heated in a liquid inactive to the base resin at the glass transition temperature and lower to the temperature lower than the boiling point of an inactive liquid by 20°C or lower. It is preferable that the inactive liquid is provided with the heat conductivity of 0.00010cal/cm.sec.°C or more. When the heat conductivity is large, heat can be transmitted quickly, and the heat can be transmitted uniformly to the base. As for the inactive liquid to be used, for instance, a fluorine liquid is used, and amoung other things, a liquid composed of perfluorocarbon is preferred. Also it is preferable to set the heating temperature lower than the glass transition point of the base resin by 10-50°C.