The SET guidelines (2013a) include the six principles. First, listed companies are required to disclose information on important matters to the investors and its affiliates excepting some special cases defined in the guidelines. Second, listed companies are required to disclose important information to the public via media and methods defined by the SET in order to publish such information to all investors extensively. Third, listed companies are required to clarify any news whether it is right or wrong as soon as possible because such news or rumors may affect stock trading or investment decisions in the shares. Fourth listed companies are required to take action on activities being conducted in a different manner. The reason is that activities being conducted in a different manner may have an effect on stock trading, these are required to disclose. Fifth,