To stably deposit palladium and to obtain a uniform continuous coating film of catalyst nuclei by incorporating a water-soluble palladium salt, a water-soluble stannous salt and a water-soluble potassium salt.

A water-soluble potassium salt is added to a water-soluble palladium salt and a water-soluble stannous salt to prepare the objective mixed catalyst. The water-soluble palladium salt is preferably the one that dissociates readily in an aq. soln. and can release a palladium ion and palladium chloride or palladium sulfate is, e.g. used. The water-soluble stannous salt is the one that dissociates readily in an aq. soln. and becomes a stannous ion and stannous chloride or stannous sulfate may be utilized. Potassium chloride, potassium sulfate or potassium hydroxide may be utilized as the water-soluble potassium salt. The molar ratio of the water-soluble potassium salt to the mixture of the water-soluble palladium salt and stannous salt in the mixed catalyst is preferably about 1:1 to 100:1.