A steam turbine plant includes high-pressure gland portions, low-pressure gland portions, a gland regulator line, and a rotor-driving steam supply line. The high-pressure gland portions supply gland steam to gaps of ends of a high-pressure turbine rotor, and thereby seal the gaps. The low-pressure gland portions supply the gland steam to gaps of ends of a low-pressure turbine rotor, and thereby seal the gaps. The gland regulator line guides the gland steam from the high-pressure gland portion to the low-pressure gland portion. The rotor-driving steam supply line branches off from the gland regulator line, and supplies some of the gland steam to a main steam flow passage in a low-pressure casing.