In order to prevent thermal deformation of a thermal insulating board and scattering of radiant heat when sintering porous glass base material, provided is a thermal insulating member is arranged on a dummy rod above a porous glass base material, which is formed by depositing glass fine particles on the outside of a starting member formed by connecting the dummy rod to at least one end of a core rod, when heating the porous glass base material to achieve sintering. The thermal insulating member comprises a cylindrical insulating cylinder; an insulating upper board connected to a top end of the insulating cylinder; an insulating lower board connected to a bottom end of the insulating cylinder; and a thermal deformation preventing member that prevents thermal deformation of at least one of the insulating cylinder, the insulating upper board, and the insulating lower board.