A heat extensible fiber comprises a conjugate fiber including a first resin component having an orientation index of 30% to 70% and a second resin component having a lower melting point or softening point than the melting point of the first resin component and an orientation index of 40% or more, the second resin component being present on at least part of the surface of the conjugate fiber in a lengthwise continuous configuration. The conjugate fiber is a heat-treated or crimped fiber and being configured to thermally extend when heated at a temperature lower than the melting point of the first resin component. The heat extensible fiber has higher heat self-extensibility than conventional extensible fibers.