The study aims to explore (1) enforcement efficiency of the Securities and Exchange Act (SEC Act) B.E. 2535 (C.E.1992) on illegal insider trading during the 10-year period (B.E. 2550-2559) based on prosecuted cases (2) enforcement efficiency of the said Act during the past 10-year period and trends for enforcement efficiency of the amended Act (SEC Act (No.5) B.E. 2559 (effective from December 12, 2017 onwards) on illegal insider trading based on perceptions of interviewees. The findings are (1) low efficiency of the SEC Act B.E. 2535 on enforcing illegal insider trading during B.E. 2550-2559 as indicated by the time-consuming proceeding of cases whereas the upward trend of cases and increasing number of defendants seem to be inconclusive indicators of law efficiency. (2) low to medium enforcement efficiency of the SEC Act B.E. 2535 and medium to high enforcement efficiency of the amended Act as perceived by the group of investors and non-investors. These perceptions somewhat differ from those of the law experts who viewed medium to high efficiency of the SEC Act B.E. 2535 and high to highest efficiency of the amended Act. The interviewees concurred that the amended Act tended to be more efficient than the original Act since the amended Act was equipped with: the clearer and broader definition of insiders, the presumption on knowers/possessors of inside information which should lead to easier and quicker proceeding of cases, and the harsher punishment measures along with the alternative civil sanction. The findings support the concept that the enforcement efficiency of law can be considered in terms of the rapidness of case proceeding and the compatibility between punishment measures and the offence type