A view through the posteromedial portal is necessary to observe the posterior portion of the knee joint such as the tibial attachment of the posterior cruciate ligament (PCL) or the posterior horn of the medial meniscus, which is barely, visible through anterior portals. Especially in arthroscopic PCL reconstruction, the arthroscopic view through the posteromedial portal is indispensable for boring a tibial bone tunnel that is correctly sited. However, it is difficult to create this portal safely without cartilaginous or meniscal damage. We have produced new guidelines to facilitate the process of creating the posteromedial portal even by inexperienced hands. With a cannulated K-wire and a cannulated rod with sharp teeth, the joint capsule of the posteromedial corner can be penetrated safely with or without viewing the posteromedial corner through the anterolateral portal. Based on our experience of using this guide system, we believe it will be a great help to many orthopaedic surgeons in performing various types of arthroscopic surgery, not only for PCL reconstruction but also for arthroscopic synovectomy.