In a capacitor comprising a pair of electrodes and a dielectric substance intervening between the two electrodes, one of the electrodes is composed of sintered niobium nitride. Preferably, the dielectric substance is composed of niobium oxide and the electrode other than the electrode composed of sintered niobium nitride is composed of an ingredient selected from electrolytes, organic semiconductors and inorganic semiconductors. This capacitor has good environmental stability and good leak current characteristics.