An expansion/contraction mechanism includes a telescopic cylinder, boom fixing means, cylinder-boom connecting means, and a hydraulic-pressure supply unit, and telescopes a plurality of booms except a base boom by telescoping the telescopic cylinder. The hydraulic-pressure supply unit includes a pneumatic-pressure source, a selector valve which selects a destination of air provided from the pneumatic-pressure source, a first pneumatic path through which first air sent from the selector valve circulates, a second pneumatic path through which second air sent from the selector valve circulates, a first pneumatic-to-hydraulic conversion unit which converts a pneumatic pressure provided by the first air to a hydraulic pressure and supplies the hydraulic pressure to a first hydraulic cylinder, and a second pneumatic-to-hydraulic conversion unit which converts a pneumatic pressure provided by the second air to a hydraulic pressure and supplies the hydraulic pressure to a second hydraulic cylinder.