The silver decoration method was applied to the Mg-11.3mol%Ni eutectic alloy to investigate hydrogen distribution in it. The plate specimens of the alloy were electrochemically hydrogen charged using NaOH aqueous solution at ambient temperature. The specimens were silver decoration treated using K[Ag(CN)2] solved KOH aqueous solution to visualize the hydrogen location as the location of the silver grain. An effect of Pt coating on the decoration process was also examined. The Pt coating has drastically decreased the undesirable precipitation of the silver grain that caused by the direct galvanic reaction between the specimen and the decoration solution. The observed hydrogen location on the hydrogen input side has not been the Mg phase but the Mg2Ni phase in the eutectic structure. The hydrogen location on the hydrogen output side has been also the Mg2Ni phase. It has been thought that hydrogen is preferentially absorbed in the Mg2Ni phase and penetrates into the specimen through the Mg2Ni phase.