To provide a reinforcing cage for a foundation pile, and a method of reinforcement arrangement for enlarging an enlarged bottom part using the reinforcing cage dispensing with a special process for diameter enlargement and simplifying structure.

The reinforcing cage for the foundation pile is provided with a straight cylinder part comprising straight cylinder part main reinforcements 1 and straight cylinder part hoops 2; enlarging part main reinforcements 1-K turnably connected to the lower ends of the straight cylinder part main reinforcements 1 by upper hinges 3; enlarging part hoops 6 connected to the intermediate parts of the enlarging part main reinforcements 1-K and formed to be variable in diameter; a hole bottom grounding interlocking ring 4 located directly below the straight cylinder part; opening/closing transmission bars 5 turnably and radially connected onto the periphery of the hole bottom grounding interlocking ring 4 at predetermined spaces; and lower hinges 3-T turnably connecting the upper end parts of the opening/closing transmission bars 5 to nearly the lower ends of the enlarging part main reinforcements 1-K. The enlarging part to which load is applied by the own weight of the whole reinforcing cage 10 for the foundation pile when grounded to the hole bottom is enlarged like a moving line of umbrella ribs to automatically arrange reinforcements at the enlarged bottom part.