Abstract: The present invention provides a method for preparing a medicament in which a pre process for the medicament comprising a step of synthesizing a montelukast sodium salt of chemical formula 1 in the specification and/or a post process for the medicament comprising a step of formulating the synthesized montelukast sodium salt is carried out in an atmosphere where a light source irradiating wavelengths of a region where the photodegradation of the montelukast sodium salt is low is provided.