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 horizontally projecting in both lateral directions of the leading end of the boom ; and jib base end engagement portions provided at respective ends of the bifurcated base end of the jib , wherein the jib base end engagement portions are U-shaped into which the jib connection shaft is allowed to be fitted, and include insertion holes for insertion of pins , the pins being intended for preventing slipping off of the jib connection shaft , characterized in that : the insertion hole in one of the jib base end engagement portions 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 (d1) 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 and a leading end of the jib is placed laterally from the boom , and at which the gap (d2) between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance (L) between the insertion hole and the U-shaped bottom portion in a direction along a central axis (O) of the jib is substantially the same as an outer diameter of the jib connection shaft . a jib connection shaft horizontally projecting in both lateral directions of the leading end of the boom ; and jib base end engagement portions provided at respective ends of the bifurcated base end of the jib , wherein the jib base end engagement portions are U-shaped into which the jib connection shaft is allowed to be fitted, and include insertion holes for insertion of pins , the pins being intended for preventing slipping off of the jib connection shaft , characterized in that : the insertion hole in one of the jib base end engagement portions 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 (d1) 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 and a leading end of the jib is placed laterally from the boom , and at which the gap (d2) between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance (L) between the insertion hole and the U-shaped bottom portion in a direction along a central axis (O) 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 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 (d1) 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 and a leading end of the jib is placed laterally from the boom , and at which the gap (d2) between the inserted pin and the jib connection shaft becomes narrower in a state where the jib is projected; and a distance (L) between the insertion hole and the U-shaped bottom portion in a direction along a central axis (O) of the jib is substantially the same as an outer diameter of the jib connection shaft . <EOS>