Lord Siva is not an ordinary living being. He is the plenary portion of the Lord, but because Lord Siva is in direct touch with material nature, he is not exactly in the same transcendental position as Lord Visnu. The difference is like that between milk and curd. Curd is nothing but milk, and yet it cannot be used in place of milk.
>>> Ref. VedaBase => SB 1.3.5
Lord Siva said: My dear son, I, Lord Brahma and the other demigods, who rotate within this universe under the misconception of our greatness, cannot exhibit any power to compete with the Supreme Personality of Godhead, for innumerable universes and their inhabitants come into existence and are annihilated by the simple direction of the Lord.
>>> Ref. VedaBase => SB 9.4.56