1. A legged robot which operates in accordance with a predetermined action sequence, comprising: input means for detecting an external factor; option providing means for providing changeable options concerning at least a portion of the action sequence; input determination means for selecting an appropriate option from among the options provided by the option providing means in accordance with the external factor detected by the input means; and action control means for performing the action sequence, which is changed in accordance with a determination result by the input determination means.