A preventing or treating agent for glaucoma is provided. This drug has a strong action of reducing intraocular pressure such that the intraocular pressure can be reduced even from the normal intraocular pressure. More specifically, a prophylactic or therapeutic agent for glaucoma, a preventing or treating agent for ocular hypertension, and eye drops containing (S)-(−)-1-(4-fluoro-5-isoquinolinesulfonyl)-2-methyl homopiperazine or its salt and phosphoric acid or its salt are provided.
