The financial investment instrument is a fundamental concept in determining the scope of the Financial Investment Services and Capital Market Act . A right or security that falls under the Act has the following legal effect: first, except for some exempt securities, it is subject to issuance and distribution disclosure obligations; second, depending on the distribution status of the financial investment instrument, it is subject to regulations on unfair trading, such as insider trading and price manipulation; third, the business of trading or underwriting a financial investment instrument is subject to the Act. How to define a financial investment instrument depends on the particular legislative objectives. The Act, contrary to the current Securities and Exchange Act, defines securities in an abstract and comprehensive way. Moving away from the institutional regulation, this enables functional regulation, which should work to strengthen investor protection. This paper outlines, from the perspective of regulatory usefulness and legal stability in the context of a comprehensive definition of financial investment instruments, ways to clarify legal vagueness. It also points out issues and possible rectifications on the scope of instruments, taking into account existing regulations on financial instruments in the U.S., the U.K., and Japan.