A connecting structure for connecting, to a connecting end portion of a first member, a second member, the connecting structure comprising the first member and the second member, wherein a first protruding portion and a first fitting portion are formed, respectively, on the connecting end portion and the second member such that, with the first protruding portion fitted to the first fitting portion, separation of the first member from the second member is prevented, wherein a second protruding portion and a second fitting portion are formed, respectively, on the connecting end portion and the second member, such that, with the first protruding portion fitted to the first fitting portion, the second protruding portion is inseparably fitted to the second fitting portion so as to keep the first protruding portion fitted to the first fitting portion, wherein the first protruding portion is fitted to the first fitting portion by displacing the first member and the second member relative to each other, first in an insertion direction in which the first protruding portion is inserted into the first fitting portion, and then in a fitting direction which is different from the insertion direction, and in which the first protruding portion is fitted to the first fitting portion, wherein the connecting structure further comprises a tunnel-shaped insertion channel connected to the first fitting portion and extending in the insertion direction, such that the first protruding portion is inserted into the first fitting portion through the insertion channel, wherein the first fitting portion is a through hole extending to an outer surface of the second member, wherein the second fitting portion is formed adjacent to the tunnel-shaped insertion channel, wherein the second protruding portion is a rib extending from the first protruding portion in a direction opposite from the insertion direction of the connecting end portion, and the second fitting portion is a groove extending along the second protruding portion, and wherein, between the tunnel-shaped insertion channel and the second fitting portion, a radially inwardly protruding partition protrusion is formed to separated the tunnel-shaped insertion channel from the second fitting portion, wherein the second protruding portion has an inclined portion of which the radially outward protruding amount decreases toward its circumferential end and wherein, while each first protruding portion is being guided toward the first fitting portion, the second protruding portion reaches the partition protrusion, wherein the distal edge of the inclined portion formed on the second protruding portion is inserted under a radially inner portion of the partition protrusion, thereby temporarily radially expanding the portion of the coupling adjacent to the second fitting portion and thus allowing smooth fitting of the second protruding portion to the second fitting portion. a first protruding portion and a first fitting portion are formed, respectively, on the connecting end portion and the second member such that, with the first protruding portion fitted to the first fitting portion, separation of the first member from the second member is prevented, wherein a second protruding portion and a second fitting portion are formed, respectively, on the connecting end portion and the second member, such that, with the first protruding portion fitted to the first fitting portion, the second protruding portion is inseparably fitted to the second fitting portion so as to keep the first protruding portion fitted to the first fitting portion, wherein the first protruding portion is fitted to the first fitting portion by displacing the first member and the second member relative to each other, first in an insertion direction in which the first protruding portion is inserted into the first fitting portion, and then in a fitting direction which is different from the insertion direction, and in which the first protruding portion is fitted to the first fitting portion, wherein the connecting structure further comprises a tunnel-shaped insertion channel connected to the first fitting portion and extending in the insertion direction, such that the first protruding portion is inserted into the first fitting portion through the insertion channel, wherein the first fitting portion is a through hole extending to an outer surface of the second member, wherein the second fitting portion is formed adjacent to the tunnel-shaped insertion channel, wherein the second protruding portion is a rib extending from the first protruding portion in a direction opposite from the insertion direction of the connecting end portion, and the second fitting portion is a groove extending along the second protruding portion, and wherein, between the tunnel-shaped insertion channel and the second fitting portion, a radially inwardly protruding partition protrusion is formed to separated the tunnel-shaped insertion channel from the second fitting portion, wherein the second protruding portion has an inclined portion of which the radially outward protruding amount decreases toward its circumferential end and wherein, while each first protruding portion is being guided toward the first fitting portion, the second protruding portion reaches the partition protrusion, wherein the distal edge of the inclined portion formed on the second protruding portion is inserted under a radially inner portion of the partition protrusion, thereby temporarily radially expanding the portion of the coupling adjacent to the second fitting portion and thus allowing smooth fitting of the second protruding portion to the second fitting portion.