The invention is to provide (i) a catalyst which does not require an activation of catalyst components by means of a calcination which has become a hindrance in the way of obtaining a catalyst having a high activity through a conventional technology and in which catalyst the compositing of vanadium with molybdenum is contemplated more than enough; ii) a method for producing the catalyst; (iii) a catalyst having an activity, especially having an activity at low temperatures and a durability both greatly increased; (iv) a catalyst compound for purifying an exhaust gas, in which compound the ratio of vanadium atom to molybdenum atom (V/Mo) is 3/2 or close thereto and which compound is expressed by the rational formula (NH₄)_xMo₂V_xO_(3x+6) wherein x is 2.8 to 3.2; and (v) a method for producing the catalyst compound through a step for reacting molybdenum oxide (MoO₃) with ammonium metavanadate (NH₄VO₃) in the co-presence of water for a prescribed period of time.