In Taiwan, the mandatory public tender offer rule was introduced in the amendment of the Securities Exchange Act in 2002. It requires bidder to make offer publicly to purchase stocks; criminal penalties are imposed on violators. In order to examine constitutionality and legitimacy of this rule, however, the legitimacy of regulatory purpose, moderateness and necessity of regulation should be further examined. Especially, if the legislator imposes criminal penalties as measure to enforce this rule, it needs to pass the tests of constitutional law and administrative law. The former relates to the constitutionality of regulatory purpose and measure; the latter relates to the effectiveness of monetary regulations enforcement. The current provision of Section 43-1 (3) of the Securities Exchange Act uses undefined legal term as a crime requirement, it might violate against the principle of legal clearness. Under the perspective of legality principle and moderation principle in criminal law, can this rule meet the rule of legislative reservation and the principle of legal clearness? Can its regulatory purpose and measure meet the proportionality principle? This study refers to the experiences of the Japanese law, introduces the concepts such as soft law governance, self-regulation and co-regulation, and further examines substantial constitutionality of this rule from the viewpoint of monetary regulation reform.