PURPOSE: To obtain a hydrogen separating medium supporting a Pd membrane without a possibility in damaging the same, by forming a membrane based on Pd covering the pore parts of the surface of a heat resistant porous body having a large number of small pores.

CONSTITUTION: A porous body having heat resistance withstanding temp. of 300°C or more, no reactivity with gas to be treated and uniform pores with a pore size of 20W30,000 is used. This porous body is used in a cylindrical or plate like shape having a thickness of 1W0.2mm. After said porous body is washed and activated, a Pd membrane is formed by chemical plating. Chemical plating is performed by a method wherein a liquid such as alcohol or water is preliminarily infiltrated in the porous body and Pd is precipitated using a plating bath based on [Pd(NH3)4]Cl2.H2O. The thickness of the membrane is about 0.001mm when a pore size is 3,000 and a required plating time is about 17hr.