This paper presents the ASVO (Automation-considered SVO) Logic that can be used for verifying authentication and key distribution protocols. The ASVO logic was designed for automatic verification, in a way to modify the SVO logic, one of the most famous authentication logics. The ASVO logic is syntactically and semantically sound, and requires relatively simple verification steps. Also we implemented the Isabelle/ASVO system which supports semi-automated verification, by using the Isabelle/Isar system.