Quine characterizes "What is there?" as the ontological question. In this paper, I tried to make clear what Quine asserts by his ontological discussion. In Quine's philosophy, ontology should be closely related to epistemology. This relation between ontology and epistemology is expressed by the reciprocal containment between both, which is followed from his naturalism. The important point of naturalism is the affirmation of a circular argument. In this paper, I considered Quine's ontology as divided into two aspects, that is, his formal considerations about ontology and his own ontological assertions, and then I pointed out that a crucial problem arises in Quine's own ontological assertions. Quine's solution to this problem is an example of how the circular argument takes a central role in his ontology. By this consideration, we can ascertain the relation between ontology and epistemology in Quine's philosophy, and the significance of ontology for the examination of Quine's philosophy.
