Reactor for preparing hydrogen cyanide by the Andrussow process, with at least one gas inlet for reactant gases mixture, a catalyst, at least one support for the catalyst and at least one outlet for the reaction products, wherein said reactor (201) comprises a cone frustum shaped casing (210) and comprises inside said casing, a gas distributor (202) located between a gas inlet (I) and a catalyst (203), said distributor comprising at least onecone frustum element (Dn) having an upper base directed towards the gas inlet.