Abstract: A tubular jacket heater having a C-wise radial cross section with an aperture extending in a parallel direction to the axis is proposed which is used to envelop and preheat a tubular lamp used, for example, in a electrophotographic copying machine. The heater is composed of an inner layer made of an electrically insulating rubbery elastomer, an outer layer also made of an electrically insulating rubbery elastomer and a flexible heater element sandwiched by and entending between the inner and the outer insulating layers. The rubbery elastomer recommended is a silicone rubber with a specified hardness.Different from conventional similar tubular jacket heaters in which the insulating layers are made of a rather rigid plastic resin such as a polycarbonate resin, the inventive heater has an excellent adaptability to the outer surface of the tubular lamp inserted therein so that a greatly improved efficiency of the heater is obtained.