The present invention provides a coating agent including hydroxyalkyl cellulose, in which a content of hydroxyalkyl group in the hydroxyalkyl cellulose is within a range of 40 to 50% by mass, and more preferably, the amount of hydroxyalkyl group in the hydroxyalkyl cellulose is within a range of 40 to 50% by mass and the viscosity of the 2% water solution at 20℃ of the hydroxyalkyl cellulose is within a range of 3.0 to 5.9mPa‧s； and a solid formulation coated with the coating agent.本發明提供一種含有羥烷基纖維素之被覆劑，其中羥烷基纖維素中之羥烷基含量在40～50質量%之範圍，較佳為羥烷基含量在40～50質量%之範圍且2%水溶液於20℃之黏度在3.0～5.9mPa‧s之範圍，以及提供一種由該被覆劑所被覆之固形製劑。
