PURPOSE: To prevent poor workmanship of shaping holes in the etching process for a metal base board as a work for a shadow mask resulting from poor performance of the etching operation caused by fine unsoluble foreign matters included in the etching solution.

CONSTITUTION: A base board of metal is subjected to an etching process with an etching solution in manufacturing a shadow mask for a color image receiving tube. The sizes and number of pieces of such foreign matters in the solution are controlled so that no more than five pieces of a size exceeding 30um are included in 10ml etching solution.