Abstract: A heat insulator for pipe lines for transportation of low temperature fluids which includes a plurality of longitudinally divided hollow cylindrical heat-insulating units assembled to tightly cover the outer periphery of the pipe lines, characterized in that each heat-insulating unit is composed of a heat-insulating panel element of a circular arc in cross section made of a foamed synthetic resin overlaid and integrally combined with an elastic sheet capable of absorbing stress caused by shrinkage of the heat-insulating panel element at low temperature and a thin metal plate placed on the outer surface of the elastic sheet and integrally combined therewith and that a sealing means is applied to the longitudinal and transverse joint contact surfaces of the adjacent heat-insulating units.