Single-incision laparoscopic surgery (SILS) has emerged as a procedure to further improve cosmetic profits and reduce the postoperative pain of multiport laparoscopic surgery. However, SILS is a difficult operation due to the limited workspace or accessibility. To improve surgical convenience, flexible surgical robots should be developed and applied to SILS for a large workspace. Flexible robots would penetrate a single incision point around the navel area and can provide a large workspace within the abdominal cavity. However, it is difficult to support the force required for surgical intervention during this process. In this study, a novel mechanism to lock the shape of a flexible joint to support the external forces during SILS is proposed. The developed shape-locking mechanism involves placing a latch between the joints; the shape locks by engagement of the latches via an electromagnetic force. The mechanism is implemented through mechanical coupling, so it can withstand large loads. Furthermore, the driving force of the mechanism is small because it only needs to engage the latch structure. This letter discusses the development of the mechanism, magnetic force simulation, and a payload experiment.