A heater (A1) includes a substrate (1), a heating resistor (2) formed on the substrate (1) and an electrode (3) electrically connected to the heating resistor (2) and containing a metal component. The heater (A1) further includes a diffusion prevention layer (4) which is held in contact with at least part of the electrode (3) and prevents the metal component from diffusing from the electrode (3). By preventing the diffusion of the metal component from the electrode (3) to the heating resistor (2), the separation of the heating resistor (2) and the electrode (3) is prevented.