A telescopic boom includes a first frame having a curved portion whose section is substantially U-shaped, and a second frame connected to the first frame so that a closed section is formed. In the telescopic boom, the U-shaped curved portion of the first frame includes a plurality of protrusion portions formed at intervals in a circumferential direction of the telescopic boom, each protrusion portion extending in a longitudinal direction of the telescopic boom and being formed to have an arc-shaped section and protrude to the outside of the first frame.