Abstract: A platinum colloid is formed by reducing platinum ions by agitating a treatment solution, while controlling temperature of the treatment solution under a reductive atmospheric condition. The solution contains one part of capacity of a platinum chloride acid solution, 400 to 500 parts of capacity of water, a non-ion based surface-active agent in an amount of 0.2 to 2 times with respect to an amount of the platinum chloride acid solution, a reducing agent in an amount of 40 to 60 times with respect to the amount of the platinum cloride acid solution, and a Ph compensating agent in an amount of 10 to 30 times with respect to the amount of the platinum chloride acid solution if the concentration thereof is 5%, to thereby adjust pH of the treatment solution to neutral or weak alkalinity.