In the naval system in classical Athens, trierarchs used in principle state hulls and sea-going equipment. In prescription, when their services were over, users had to return them in good condition. Those did not do so were recorded in naval documents as a debtor to the State. The object of this treatise is to make clear the actual situation of debts and their collection, and to consider the historical background. In order to attain this aim, naval inscriptions (IGII^2 1604-1632) have been mainly examined. They record details of the administration of the Athenian navy over about 55 years from the 370s to the 320s B.C.. This examination suggests important points in regard to the controlling power of the State over its wealthy citizens and their social power. From an examination of the inscriptions, the following points can be infered. (1)Debts tended to reach long terms, and each debtor was inclined to have plural and long-term debts. This confirms a view generaly pointed out by many scholars; which shows both a tendency to delay repayment of debts and. inactivity of collection in the navy. (2)However, except for the concentrated collection of debts in the first half of 350s., collection seems to have become active at latest after the middle of 340s B.C., this can be thought to have been greatly influenced by external circumstances: formed the preparations at war against Macedon in the second half of 340s, and security of sea trade routes after the battle of Chaeronea, above all measures for corn supply in the face of food crises. (3)The sum of collected money at times reached more than 10 talents. With 10 talents one could supply 26-27 sets of sea-going equipment. Therefore, collections were of considerable importance for consolidating the navy as a facfor to make sea trade active. (4)It is not necessarily clear whether the intensification of collecting debts had a direct relation to the policies of Eubulus and Lykurgus. But at least there is a possibility that their policies had some influence on it. The author concludes from the above that the character of administration of the Athenian navy in the first half of the fourth century B.C. differs significantly from that of the second. And in its background there seems to have been urgent and difficult circumstances : Social War and relations with Macedon and food crises etc.. Only in the face of such situations was debt collection intensified. It also gives an interesting clue to considering the power of the wealthy citizens in those days.