An optical system for endoscopes comprising a light source and a condenser optical system for focusing the light emitted from said light source, and adapted so as to lead the light emitted from said light source to be incident through said condenser optical system onto light guides having different NA's, said condenser optical system being equipped with a means capable of varying or adjusting NA of the light emerging from said condenser optical system in conjunction with NA of a light guide selected for use. This optical system for endoscopes permits matching NA of the emerging light bundle with the NA of the selected light guide by using the adjusting means, thereby improving light distribution characteristic and transmission efficiency.