To provide a sensor holding mechanism for an inspection pig which can climb over the inside protrusion part of a circumferential weld or other obstacles and which is not damaged.

The right-side tip of a bracket 11 is situated at the inner side from the maximum part out of the inside protrusion part 12 of a circumferential weld, and its outer face side is inclined gradually toward the side of a magnetic sensor holder 6 from the tip part. When the bracket 11 comes into contact with the inside protrusion part 12 of the circumferential weld during an inspection, it is brought surely into contact with the inclined part. Then, the outer face of the bracket 11 which is brought into contact with it and the inside protrusion part 12 of the circumferential weld are slid smoothly, and a force which presses the sensor holder 6 backward does not become large. In addition, a component of force which presses the sensor holder 6 inward is generated, a parallel link mechanism 9 acts due to the force, and the sensor holder 6 is moved inward. In this manner, the sensor holder 6 can climb over the inside protrusion part 12 of the circumferential weld smoothly.