A jib connection structure being a connection structure between a leading end of a boom and a base end of a jib, the jib connection structure comprising: a jib connection shaft (31, 32) horizontally projecting in both lateral directions of the leading end of the boom; and jib base end engagement portions (31, 32) provided at respective ends of the bifurcated base end of the jib, wherein the jib base end engagement portions (31, 32) are U-shaped into which the jib connection shaft (31, 32) is allowed to be fitted, and include insertion holes (31h, 32h) for insertion of pins (33, 34), the pins (33, 34) being intended for preventing slipping off of the jib connection shaft (31, 32), characterized in that: the insertion hole in one of the jib base end engagement portions (31, 32) is placed laterally outwardly from a U-shaped bottom portion; the laterally outwardly placed insertion hole is located at a position at which a gap is formed between the inserted pin and the jib connection shaft in a state where the jib connection shaft is fitted into the jib base end engagement portions (31,32) and a leading end of the jib is placed laterally from the boom, and at which the gap between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance between the insertion hole and the U-shaped bottom portion in a direction along a central axis of the jib is substantially the same as an outer diameter of the jib connection shaft. a jib connection shaft (31, 32) horizontally projecting in both lateral directions of the leading end of the boom; and jib base end engagement portions (31, 32) provided at respective ends of the bifurcated base end of the jib, wherein the jib base end engagement portions (31, 32) are U-shaped into which the jib connection shaft (31, 32) is allowed to be fitted, and include insertion holes (31h, 32h) for insertion of pins (33, 34), the pins (33, 34) being intended for preventing slipping off of the jib connection shaft (31, 32), characterized in that: the insertion hole in one of the jib base end engagement portions (31, 32) is placed laterally outwardly from a U-shaped bottom portion; the laterally outwardly placed insertion hole is located at a position at which a gap is formed between the inserted pin and the jib connection shaft in a state where the jib connection shaft is fitted into the jib base end engagement portions (31,32) and a leading end of the jib is placed laterally from the boom, and at which the gap between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance between the insertion hole and the U-shaped bottom portion in a direction along a central axis of the jib is substantially the same as an outer diameter of the jib connection shaft. the insertion hole in one of the jib base end engagement portions (31, 32) is placed laterally outwardly from a U-shaped bottom portion; the laterally outwardly placed insertion hole is located at a position at which a gap is formed between the inserted pin and the jib connection shaft in a state where the jib connection shaft is fitted into the jib base end engagement portions (31,32) and a leading end of the jib is placed laterally from the boom, and at which the gap between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance between the insertion hole and the U-shaped bottom portion in a direction along a central axis of the jib is substantially the same as an outer diameter of the jib connection shaft.