PURPOSE: To develop a method of etching a compound semiconductor primarily compounded of PbS which enables high accuracy selection etching to be carried out and a flat surface free of roughness or discoloration to be created after the etching.

CONSTITUTION: This etching method includes a stage (a) which covers a specified region of a compound semiconductor primarily made of PbS with a metal layer, such as gold and a stage (b) which etches the surface portion not covered with the gold by means of an alkali etching solution which includes a complexing agent which forms hydrogen peroxide, Pb and complex (such as alkali metal salt of tartaric acid and sodium hydroxide) and alkali hydroxide. This method is suitable to mesa etching. There are formed cladding layers 3 and 5 made of PbCdSSe on a PbS substrate 1 with a PbS active layer sandwiched, which are provided with a contact layer 6 and an electrode 7 and useful to the preparation of mesa stripe type laser diode in double heterostructure.