A fault diagnosis device conducts a fault diagnosis in a temperature sensor. At starting an internal combustion engine, a thermal equilibrium condition may be established and temperature deviations of temperatures detected by at least two reference temperature sensors from each other may be equal to or less than a predetermined value, with the temperatures detected by the reference temperature sensors being greatly deviated from one detected by a temperature sensor being diagnosed. In this case, if the temperatures detected by the reference temperature sensors do not drop by a predetermined temperature or more from starting the engine until a predetermined time elapses, the fault diagnosis device determines that the temperature sensor being diagnosed is faulty.