To provide a telescopic boom of a walking crane without making mutual boom interfere, even if large elastic deformation is generated in the telescopic boom 30 in crane work, by arranging a slide plate 42 in a part having risk of generating interference.

This telescopic boom 30 of the walking crane is constituted by successively slidingly fitting and inserting an inside boom in an outside boom. The slide plate 42 capable of slidingly contacting with an inner surface of the outside boom 31, is arranged on an inside boom outside surface being an intermediate area in the longitudinal direction of the inside boom 32.