Provided is a preparation of ramosetron which is stable under irradiation with light. The solid pharmaceutical composition of the present invention can provide a stable preparation by blending a compound absorbing light having a specific wavelength with ramosetron which is unstable, usually under irradiation with light, or a pharmaceutically acceptable salt thereof. Particularly, this technique is useful because it is adaptable to a preparation containing ramosetron or a pharmaceutically acceptable salt thereof at a low content or an orally disintegrating tablet containing ramosetron or a pharmaceutically acceptable salt thereof. Also, the present invention relates to a method for stabilizing a solid pharmaceutical composition of ramosetron or a pharmaceutically acceptable salt thereof, which is characterized by blending a compound having characteristics of absorbing light having a specific wavelength.
