Provided is a heater unit having a fine wire sheath heater with improved reliability. This heater unit comprises: a first substrate and a second substrate which are bonded to one another; a groove which is provided to the bonding surface of the first substrate and/or the second substrate; and a sheath heater positioned on the inside of the groove. The sheath heater is equipped with a metal sheath;a heating wire which is positioned inside the metal sheath with a gap therebetween, the heating wire being ribbon-shaped, and being positioned so as to rotate about the axial direction of the metal sheath; an insulating material positioned in the gap; and connecting terminals which are positioned at one end of the metal sheath, each connecting terminal being electrically connected to one end of the heating wire.