disclosed is a process of preparing a catalyst supporting highly metal particles comprising , in the presence of carbon monoxide in a solution containing a metal - containing ion and a support on which metal particles produced by reduction of the metal - containing ion are supported , reducing by means of a reductant the metal - containing ion to the corresponding metal particles which are then supported on the support . according to the process of the present invention , because of the presence of the carbon monoxide which is supposed to have a function of depressing the deposition of a new metal particle present around the metal particles supported on the support , that is , the growth of the metal particles and another function of preventing the agglomeration among the metal particles supported , the catalyst of excellent properties having the metal particles supported and monodispersed on the support with the narrow particle size distribution can be prepared of which an average particle size is not more than 20 å . after the preparation of the catalyst , the carbon monoxide can be easily taken out from the system so that no harmful effects are produced by the employment of the carbon monoxide .