A hole expansion forming device comprising: a cutting portion main body formed to have a diameter to be housed in a joint hole and including a cutting blade tool movable in a range from a housed position at which a blade is housed within the diameter to an extended position at which the blade projects out of the diameter, an advancing and retracting member that is moved in an axial direction by an external force and rotates integrally in an axis circumferential direction, a link mechanism connecting the cutting blade tool and the advancing and retracting member, the link mechanism being adapted to convert advancing and retracting movement of the advancing and retracting member into movement of the cutting blade tool, and a housing body whose sectional outer shape is substantially circular; a tip portion bearing that is provided on a tip end side of the cutting portion main body and serves as a tip end side bearing by being housed in an inner portion of the joint hole; and a torque transmitting element to transmit a torque in the axis circumferential direction to the advancing and retracting member, characterised in that a cylinder having about the same diameter as that of the housing body is formed on a rear side of the housing body, in that the advancing and retracting member has, on a rear portion side, a large-diameter portion having a slightly smaller diameter than an inner diameter portion of the cylinder, and on a tip portion side, a small-diameter portion having a slightly smaller diameter than the inner diameter of a coil spring, which is housed in a front side of the inside of the inner diameter portion of the cylinder, in that a through hole penetrates through a front side of the cylinder so as to communicate with a housing space, which is formed to penetrate through both sides of an outer circumferential surface including both ends of a diametrical line of the housing body, and in that the small-diameter portion is inserted to penetrate through the coil spring and the through hole, by sliding the large-diameter portion along the inner diameter portion and sliding the small-diameter portion along the through hole, the advancing and retracting member can advance and retract while being subjected to a biasing force of the coil spring. a cutting portion main body formed to have a diameter to be housed in a joint hole and including a cutting blade tool movable in a range from a housed position at which a blade is housed within the diameter to an extended position at which the blade projects out of the diameter, an advancing and retracting member that is moved in an axial direction by an external force and rotates integrally in an axis circumferential direction, a link mechanism connecting the cutting blade tool and the advancing and retracting member, the link mechanism being adapted to convert advancing and retracting movement of the advancing and retracting member into movement of the cutting blade tool, and a housing body whose sectional outer shape is substantially circular; a tip portion bearing that is provided on a tip end side of the cutting portion main body and serves as a tip end side bearing by being housed in an inner portion of the joint hole; and a torque transmitting element to transmit a torque in the axis circumferential direction to the advancing and retracting member, characterised in that a cylinder having about the same diameter as that of the housing body is formed on a rear side of the housing body, in that the advancing and retracting member has, on a rear portion side, a large-diameter portion having a slightly smaller diameter than an inner diameter portion of the cylinder, and on a tip portion side, a small-diameter portion having a slightly smaller diameter than the inner diameter of a coil spring, which is housed in a front side of the inside of the inner diameter portion of the cylinder, in that a through hole penetrates through a front side of the cylinder so as to communicate with a housing space, which is formed to penetrate through both sides of an outer circumferential surface including both ends of a diametrical line of the housing body, and in that the small-diameter portion is inserted to penetrate through the coil spring and the through hole, by sliding the large-diameter portion along the inner diameter portion and sliding the small-diameter portion along the through hole, the advancing and retracting member can advance and retract while being subjected to a biasing force of the coil spring.