PURPOSE: To provide a method for producing a noble metal alloy catalyst by which the control of an alloy compsn. is facilitated and a desired alloy compsn. can be obtd. by increasing the rate of carrying of a heavy metal by the control of the temp. of a plating soln. during a reaction.

CONSTITUTION: A noble metal or noble metal alloy catalyst carried on a powdery carrier is dispersed in hot water to prepare a catalyst slurry. A plating soln. of pH 10.5-13.0 contg. a heavy metallic salt, a chelating agent. and a reducing agent is prepd. and put in the catalyst slurry and electroless plating is carried out at a low temp. The resultant noble metal or noble metal alloy catalyst plated with the heavy metal and carried on the carrier is washed, dried and subjected to alloying treatment by heating in a required atmosphere. A reduction reaction of the heavy metallic salt is allowed to proceed slowly and a point of time when the reduction reaction finishes practically and teaching does not occur yet is easily ascertained.