To provide a gear case excellent in slidability, mechanical strength, and dimensional stability while reducing the amount of inorganic filler used, especially the amount of fibrous inorganic filler used; and to provide a gear unit having the gear case.

A polyacetal resin composition for a gear case is used which comprises: 70 to 85 mass% of polyacetal; 5 to 25 mass% of a fibrous inorganic filler; and 5 to 20 mass% of a non-fibrous inorganic filler, wherein the total of a content of the fibrous inorganic filler and a content of the non-fibrous inorganic filler is 30 mass% or lower.