This study proposes a self-locking-type expansion mechanism for in-pipe robots. Previously, we proposed a highspeed locomotion mechanism using pneumatic hollow-shaft actuators; however, this mechanism lacked holding force and could not pass through a bent pipe. The proposed mechanism generates a large holding force and can easily pass through a bent pipe by invoking a self-locking phenomenon. We conceptualize and design the novel expansion mechanism and introduce its associated mathematical model to formulate the holding force and mechanism design. The characteristics and capabilities of the mechanism are elucidated by experiments. From the experimental results, we optimize the applied pressure and the design of the mechanism. The proposed mechanism generates a maximum holding force of 69.7 N, which is 5.2 times higher than that of the previous mechanism, and drastically improves the robot's bent-pipe-passing capability. Finally, the performance of this mechanism is confirmed in a simulated pipe test. In this trial, a robot equipped with the proposed mechanism smoothly and steadily moves through complex pipe configurations, including the vertical and bent pipes.