A coating agent containing a hydroxyalkyl cellulose having a hydroxyalkyl group content of not less than 50% by mass and not more than 60% by mass with respect to the total mass of the hydroxyalkylcellulose in an amount of not less than 1% by mass and not more than 7% by mass with respect to the total mass of the coating agent is obtained. The coating agent is sprayed in a predetermined manner and dried to obtain a solid preparation.히드록시알킬기의 함유량이 히드록시알킬 셀룰로오스의 총 질량에 대해 50질량％ 초과 60질량％ 이하의 히드록시알킬 셀룰로오스를 코팅제의 총 질량에 대해 1질량％ 이상 7질량％ 이하 함유하는 코팅제를 얻는다. 당해 코팅제를 소정에 분무하고 건조시켜 고형 제제를 얻는다.
