A container-securing device comprising: a flange located on an outer side of a groove hole in a corner casting of a container; a support pillar provided in the flange for fitting in a mouth of the groove hole, wherein the support pillar is formed with vertical surface portions in parallel to the long side of the mouth; a fitting portion provided in the support pillar for fitting into a hollow of the groove hole; and the fitting portion is provided with a movable protrusion which is pivotable on a horizontal shaft and is urged to protrude laterally out of a vertical surface portion of the support pillar, wherein the movable protrusion has a first sloped surface, a second sloped surface and a convex peak between these sloped surfaces; and at least one of the first sloped surface and the second sloped surface is curved or composed of two or more flats, wherein the first sloped surface comes in contact with a far corner region at a far end of the mouth when the container is lowered for locking or lifted for unloading and wherein the second sloped surface comes in contact with an entrance corner at an entrance side of a mouth vertical surface region when the container is lowered for locking or lifted for unloading, and a fixed protrusion has a first sloped surface, a second sloped surface and a convex peak between these sloped surfaces; and at least one of the first sloped surface and the second sloped surface is curved or composed of two or more flats, wherein the first sloped surface makes contact with the far corner region at the far end of the mouth when the container is lifted for unloading and wherein the second sloped surface makes contact with the entrance corner at the entrance side of the mouth vertical surface region when the container is lowered for locking, the fixed protrusion has its most protruding region protruding from one of the vertical surface portions and the movable protrusion has its most protruding region protruding from the other of the vertical surface portions, and the one in the movable protrusion is closer to the flange than the one in the fixed protrusion, and a locked state is achieved when the flange comes in contact with an outer surface region of the corner casting, and the first sloped surface of the movable protrusion comes in contact with a far corner region which is at a far end of the mouth of the corner casting at some point when the movable protrusion pivots. a fitting portion provided in the support pillar for fitting into a hollow of the groove hole; and the fitting portion is provided with a movable protrusion which is pivotable on a horizontal shaft and is urged to protrude laterally out of a vertical surface portion of the support pillar, wherein the movable protrusion has a first sloped surface, a second sloped surface and a convex peak between these sloped surfaces; and at least one of the first sloped surface and the second sloped surface is curved or composed of two or more flats, wherein the first sloped surface comes in contact with a far corner region at a far end of the mouth when the container is lowered for locking or lifted for unloading and wherein the second sloped surface comes in contact with an entrance corner at an entrance side of a mouth vertical surface region when the container is lowered for locking or lifted for unloading, and a fixed protrusion has a first sloped surface, a second sloped surface and a convex peak between these sloped surfaces; and at least one of the first sloped surface and the second sloped surface is curved or composed of two or more flats, wherein the first sloped surface makes contact with the far corner region at the far end of the mouth when the container is lifted for unloading and wherein the second sloped surface makes contact with the entrance corner at the entrance side of the mouth vertical surface region when the container is lowered for locking, the fixed protrusion has its most protruding region protruding from one of the vertical surface portions and the movable protrusion has its most protruding region protruding from the other of the vertical surface portions, and the one in the movable protrusion is closer to the flange than the one in the fixed protrusion, and a locked state is achieved when the flange comes in contact with an outer surface region of the corner casting, and the first sloped surface of the movable protrusion comes in contact with a far corner region which is at a far end of the mouth of the corner casting at some point when the movable protrusion pivots.