The present invention provides a fuel cell unit, fuel cell unit array, fuel cell module and fuel cell system that can achieve a reduction in size and costs. The fuel cell unit of the present invention has a PIN structure comprising a fuel electrode (11) formed of a p-type semiconductor, an oxygen electrode (13) formed of an n-type semiconductor, and an interlayer (12) formed of an intrinsic semiconductor lying between the fuel electrode (11) and the oxygen electrode (13); whereinthe fuel electrode (11) and the oxygen electrode (13) are porous and have fluid permeability;a metal-based catalyst layer is formed on the inner surfaces of pores in the porous portion;the interlayer (12) is porous so that hydrogen ions generated at the fuel electrode (11) can pass therethrough, but electrons are blocked;hydrogen-containing fuel is supplied to the surface of the fuel electrode (11) and an oxygen-containing oxidizing fluid is supplied to the surface of the oxygen electrode (13); andconnecting terminals each electrically connected to the fuel electrode (11) and the oxygen electrode (13).