Reliance principle (信頼の原則) limits the scope of criminal negligence. In medical cases, this principle protects medical practitioners from excessive criminal accusations. On the other hand, if this principle is easily accessed, the medical practitioner will possess improper privilege which impairs patients’ right to live and healthy well-being. Therefore, it is necessary to set appropriate preconditions for the application of the reliance principle to protect both medical practitioners’ interests and patients’ interests. However, the predominant opinion on the prerequisites of the reliance principle in Japan has irrationalities. The standards of clear division of labor (分業の原則) and substantive trust (実質的信頼関係) does not suit the current health care system. Also, the significance of no supervisory duty (監督義務) was overlooked. This paper intended to fill the gap by using legal case studies and theories analysis to show that (1) in general medical cases, clear division of labor is not a reason to negate the reliance principle; (2) the standards of substantive trust can be replaced by the compliance with the medical norms; (3) supervisory duty negates the reliance principle. Additionally, for the rationality of the duty of foreseeability (予見義務) and avoidance (回避義務), the reliance principle is located in the position of foreseeability(予見可能性). In summary, this paper aims at improving the preconditions of the reliance principle to promote the usage of the reliance principle in judicial practice and to clear the position of the reliance principle in the theoretical system of crime.