the reason why the allyl alcohol and the hydrazinc hydrate are added to nitric acid solution of the dinitrodiamine platinum is that platinum containing colloid is formed which is then supported on the support if the allyl alcohol is added and then the hydrazinc hydrate is added to the platinum solution prepared by dissolving the crystal of the dinitrodiamine platinum in the nitric acid without the addition of the catalyst support material yellowish brown colloid like particles are formed the platinum containing rate of the particles in this case is about 60 in weight upon analysis these particles are not platinum metal colloid and are considered to be a compound of the dinitrodiamine platinum the allyl alcohol and the hydrazinc hydrate though its structure is unknown for this reason the initial addition of the allyl alcohol and the hydrazinc hydrate to the nitric acid solution of the dinitrodiamine platinum containing the support material enables to finely support the above compound on the support and by thermally treating it in a hydron containing flow a catalyst which supports highly monodispersed platinum particles of which a particle diameter is less than about 20 u00e5 can be obtained as a support of the present invention any material which is stable in a reduction atmosphere and in a reaction condition in which the resulting catalyst is employed can be utilized the most desirable support material is carbon black most parts of the allyl alcohol and the hydrazinc hydrate employed in the invention may removed by.