Method of manufacturing optical semiconductor device and optical semiconductor device

A method of manufacturing an optical semiconductor device, comprising:

• a mounting step of placing an optical semiconductor chip on a package substrate made of ceramic;
• a storing step of storing the package substrate after the mounting step in a first dry atmosphere;
• a placing step of subjecting the optical semiconductor chip on the package substrate to a second dry atmosphere and placing a light transparent window on a joint portion of the package substrate with a joint material therebetween;
• and
• a sealing step of joining the joint portion and the light transparent window with the joint material in a low oxygen concentration atmosphere having an oxygen concentration of 1 vol % or less, thereby encapsulating the optical semiconductor chip in a confined space formed by the package substrate and the light transparent window,
• wherein the confined space after the sealing step has a moisture concentration of 1000 ppm or less and an oxygen concentration of 3 vol % or less.