A telescopic boom extension device comprising: a telescopic boom that includes a base boom, an intermediate boom (22 to 24) inserted into the base boom, and a top boom inserted into the intermediate boom (22 to 24), one of the booms (20 to 24) adjacent to each other being placed slidably relative to the other boom; a single extension cylinder that includes a cylinder tube and a cylinder rod and that is built into the telescopic boom along a longitudinal direction of the booms (20 to 24) while the cylinder rod is coupled to the base boom; a cylinder-boom coupling mechanism that includes a first hydraulic actuator configured to engage selectively with the top boom or the intermediate boom (22 to 24) to couple the engaged boom to the cylinder tube; an inter-boom fixing mechanism that includes a second hydraulic actuator configured to couple the adjacent booms to regulate relative sliding of the booms and to decouple specific coupled booms when necessary; and a driving mechanism configured to drive the cylinder-boom coupling mechanism and the inter-boom fixing mechanism, wherein the driving mechanism includes: a hydraulic supply part that is provided at the cylinder tube to supply operating oil selectively to the first hydraulic actuator or the second hydraulic actuator; and characterized by further comprising a drive source generation part that includes a pneumatic supply part configured to supply a pneumatic pressure and that is configured to generate the operating oil under a predetermined pressure at the hydraulic supply part based on the pneumatic pressure. a telescopic boom that includes a base boom, an intermediate boom (22 to 24) inserted into the base boom, and a top boom inserted into the intermediate boom (22 to 24), one of the booms (20 to 24) adjacent to each other being placed slidably relative to the other boom; a single extension cylinder that includes a cylinder tube and a cylinder rod and that is built into the telescopic boom along a longitudinal direction of the booms (20 to 24) while the cylinder rod is coupled to the base boom; a cylinder-boom coupling mechanism that includes a first hydraulic actuator configured to engage selectively with the top boom or the intermediate boom (22 to 24) to couple the engaged boom to the cylinder tube; an inter-boom fixing mechanism that includes a second hydraulic actuator configured to couple the adjacent booms to regulate relative sliding of the booms and to decouple specific coupled booms when necessary; and a driving mechanism configured to drive the cylinder-boom coupling mechanism and the inter-boom fixing mechanism, wherein the driving mechanism includes: a hydraulic supply part that is provided at the cylinder tube to supply operating oil selectively to the first hydraulic actuator or the second hydraulic actuator; and characterized by further comprising a drive source generation part that includes a pneumatic supply part configured to supply a pneumatic pressure and that is configured to generate the operating oil under a predetermined pressure at the hydraulic supply part based on the pneumatic pressure. a hydraulic supply part that is provided at the cylinder tube to supply operating oil selectively to the first hydraulic actuator or the second hydraulic actuator; and characterized by further comprising a drive source generation part that includes a pneumatic supply part configured to supply a pneumatic pressure and that is configured to generate the operating oil under a predetermined pressure at the hydraulic supply part based on the pneumatic pressure.