Sustained release formulation containing tacrolimus or its hydrate is provided. The time (T63.2%) required for 63.2% of the maximum amount of tacrolimus or its hydrate to be dissolved is 0.7 to 15 hours. The time is measured in accordance to the Japanese Pharmacopoeia, the 13-th edition, Dissolution Test, No. 2 (Puddle method, 50 rpm) using an aqueous 0.005% hydroxypropyl cellulose solution. This aqueous test solution is adjusted to pH 4.5, accordingly. The formulation further comprise a solid base which is a water-soluble polymer and/or wax. The formulation is in the form of a powder, fine powder, granule, tablet or capsule. Furthermore, the formulation is administered to a patient once a day for preventing organ or tissue rejection by transplantation or autoimmune diseases.