A heather unit including a small diameter sheath heater with improved reliability is provided. The heater unit includes a first substrate having a first joint surface and a second substrate having a second joint surface being joined together, a groove arranged on at least one of the first joint surface or the second joint surface, and a sheath heater arranged inside the groove. The sheath heater includes a metal sheath, a heating wire having a band shape, the heating wire arranged with a space within the metal sheath so as to rotate with respect to an axis direction of the metal sheath, an insulating material arranged in the space, and connection terminals arranged at one end of the metal sheath, the connection terminals electrically connected with both ends of the heating wire respectively.