The TPM(Trusted Platform Module) is a hardware chip to support a trusted computing environment. A rightful user needs a command authorization process in order to use principal TPM commands. To get command authorization from TPM chip, the user should perform the OIAP(Object-Independent Authorization Protocol) or OSAP(Object-Specific Authorization Protocol). Recently, Chen and Ryan alerted the vulnerability of insider attack on TPM command authorization protocol in multi-user environment and presented a countermeasure protocol SKAP(Session Key Authorization Protocol). In this paper, we simulated the possibility of insider attack on OSAP authorization protocol in real PC environment adopted a TPM chip. Furthermore, we proposed a novel countermeasure to defeat this insider attack and improve SKAP's disadvantages such as change of command suructures and need of symmetric key encryption algorithm. Our proposed protocol can prevent from insider attack by modifying of only OSAP command structure and adding of RSA encryption on user and decryption on TPM.