Boom extending and retracting apparatus of a crane

A boom extending and retracting apparatus of a crane, comprising:

• a telescopic boom including a plurality of boom members that can slide with respect to each other;
• a telescopic cylinder configured to extend and retract the telescopic boom by sliding the plurality of boom members, the telescopic cylinder including:
• a cylinder tube having an internal space filled with fluid;
• and
• a piston rod having piston dividing the internal space of the cylinder tube into an extension-side oil chamber and a retraction-side oil chamber, the piston rod moving along an inner surface of the cylinder tube to extend and retract the telescopic cylinder;
• a fluid circuit including a normal fluid circuit and a differential fluid circuit, the normal fluid circuit injecting fluid only into the extension-side oil chamber of the telescopic cylinder, and the differential fluid circuit injecting the fluid into the extension-side oil chamber while the fluid in the retraction-side oil chamber can be moved to the extension-side oil chamber;
• a controller configured to choose one of the normal fluid circuit and the differential fluid circuit, in order to extend and retract the telescopic boom;
• and
• an operating lever turned from a neutral position in order to extend the telescopic boom,
• wherein after the operating lever is turned to an extension side and the controller starts extending the telescopic boom using the differential fluid circuit, when the operating lever is turned to the extension side again via the neutral position, the controller chooses the normal fluid circuit to extend the telescopic boom using the normal fluid circuit.