A heater includes: a substrate including metal and having a shape extending in one direction; a first insulating part provided on a first surface of the substrate; a first heating element provided on the first insulating part and extending along a longitudinal direction of the substrate; a first protection part provided on the first insulating part and covering the first heating element; a second insulating part provided on a second surface of the substrate facing the first surface; a second heating element provided on the second insulating part and extending along the longitudinal direction of the substrate; and a second protection part provided on the second insulating part and covering the second heating element. A length of the second heating element is different from a length of the first heating element in the longitudinal direction of the substrate.