A cable tie locking structure comprising a cable tie including a strip of plastically deformable strap portion having a first locking hole that extends through in a thickness direction at a proximal end thereof, and a head portion provided so as to surround the proximal end and a distal end of the strap portion that are overlapped upon one another, and secured to the proximal end, the head portion including a second locking hole and a through hole at positions respectively facing the proximal end and the distal end of the strap portion to coincide with the first locking hole of the strap portion, the locking structure securing the distal end of the strap portion to the proximal end thereof and to the head portion so as to keep the cable tie wrapped around and holding together material to be bundled, characterized in that: the first locking hole is formed in a shape with a first portion located nearer to the proximal end of the strap portion having a larger width in a short-side direction of the strap portion than that of a second portion extending along a longitudinal direction of the strap portion, and has a first inner wall surface extending in a direction substantially orthogonal to the longitudinal direction of the strap portion at the first portion located nearer to the proximal end of the strap portion; the second locking hole is formed in a shape with a third portion located nearer to the proximal end of the strap portion having a larger width in the short-side direction of the strap portion than that of a fourth portion extending along the longitudinal direction of the strap portion, and has a second inner wall surface extending in a direction substantially orthogonal to the longitudinal direction of the strap portion at the third portion located nearer to the proximal end of the strap portion; the distal end of the strap portion has a protruded portion that protrudes through the first locking hole and into the second locking hole at a point where the first locking hole, the second locking hole, and the through hole coincide with each other; and the protruded portion has an engagement surface that faces the first inner wall surface and the second inner wall surface and that extends along these inner wall surfaces (18, 19), so that the protruded portion can engage with the first inner wall surface and the second inner wall surface at this engagement surface. the first locking hole is formed in a shape with a first portion located nearer to the proximal end of the strap portion having a larger width in a short-side direction of the strap portion than that of a second portion extending along a longitudinal direction of the strap portion, and has a first inner wall surface extending in a direction substantially orthogonal to the longitudinal direction of the strap portion at the first portion located nearer to the proximal end of the strap portion; the second locking hole is formed in a shape with a third portion located nearer to the proximal end of the strap portion having a larger width in the short-side direction of the strap portion than that of a fourth portion extending along the longitudinal direction of the strap portion, and has a second inner wall surface extending in a direction substantially orthogonal to the longitudinal direction of the strap portion at the third portion located nearer to the proximal end of the strap portion; the distal end of the strap portion has a protruded portion that protrudes through the first locking hole and into the second locking hole at a point where the first locking hole, the second locking hole, and the through hole coincide with each other; and the protruded portion has an engagement surface that faces the first inner wall surface and the second inner wall surface and that extends along these inner wall surfaces (18, 19), so that the protruded portion can engage with the first inner wall surface and the second inner wall surface at this engagement surface.