"How and to what extent did religion regulate the acts of the ancient Greeks?" This is the problem the author investigates in this paper. The most useful documents for this investigation are the works of Xenophon and the Attic orators, since they reflect the average religious mentality of the people. Studying these works, the author has found three main factors which strongly regulated the people's behaviour : oaths, oracles (with other divinations), and views about life after death. Concerning each factor he draws the following conclusions. (1)Oaths : Though referring to oaths frequently, the Greeks seem to have doubted divine punishment for perjury. But they could not break their oaths freely, because it caused great blame among the people. Many examples show that they could break them most easily with political intentions. (2)Oracles : While many examples show the people determined their acts according to oracles, some examples deny their blind faith in them. The author examines two examples which show their neglect of oracles (Xen. Hell. V. 4. 47 ; Aisch. III. 130-1 with scholia) and concludes that (i)they were forced to act as if they believed oracles, (ii)they distrusted them in practice, and (iii)those who distrusted them most were politicians and those who distrusted them least were soldiers (the author uses the terms, politicians and soldiers, as ideal types). (3)Views about life after death : The idea that no one can know about life after death may be pointed out as the one which regulate people's behaviour most effectively. Because of this idea they scarcely did or made anything for life after death. This idea also caused them to keep traditional rites. While admitting this idea, they considered the possible picture of life after death, from which the thought that an honourable death was preferable to a disgraceful life appeared. This thought also regulated people's (especially soldiers') acts, but not to a high degree. While admiring this thought, politicians themselves were hardly regulated by it. Through these investigations politicians are found to be unique. Their uniqueness must have been supported by the development of orations. The technique of orations developed to such an extent that orators could say anything including a matter of the gods, which made the intention of gods uncertain and thus weakened religious regulation. Besides this situation, owing to the continual political crises during this century people had to commit themselves to politics to some extent. As a result people came to stand more and more aloof from religious regulation, following the example of politicians.