At least one basic precipitant selected from the group consisting of NaOH, KOH, NH 4 OH, NH 3 , NH 4 HCO 3 and (NH 4 ) 2 CO 3 is added to an indium salt solution, which contains 0.1 to 3 M of indium, in an adding time of not longer than 24 hours, while the solution is maintained at a temperature of 5 to 95 DEG C until the equivalent of the basic precipitant reaches an equivalent of 0.5 to 3. Then, a precipitate obtained from the solution by a solid-liquid separation is dried and calcined at a temperature of 570 to 780 DEG C in an non-oxidizing atmosphere which contains ammonia gas and watervapor.