A heater for a pipe, includes a heater body part having a longitudinal cutting line formed in the entire thickness section to form a pair of longitudinal cutting planes in the entire length section, a heating wire installed in the heater body part, and a restraint operating part installed at the heater body part and restraining the pair of longitudinal cutting planes not to be spaced apart from each other. Each of the longitudinal cutting planes includes: a longitudinal outer cutting area disposed in a thickness direction from an outer surface of the heater body part, a longitudinal intermediate cutting area extending from the longitudinal outer cutting area in a circumferential direction of the heater body part, and a longitudinal inner cutting area extending from the longitudinal intermediate cutting area to reach the inner surface of the heater body part in the thickness direction of the heater body part.