The present invention provides a biped (two-footed) walking mobile system, its walk controller, and walk control method therefore, which are to realize enhancing an walk stability, as well as a consumed energy saving. A walk controller (30) of a biped walking mobile system forms a gait data by a gait forming part (33) based on parameters from a gait stabilizing part (32), and drive-controls drive means of respective joint portions (15L, 15R-20L, 20R) of each leg portion based on said gait data. In this case, the walk controller (30) is so constituted as to selectively witch a powered mode to conduct ordinary drive-control and a passive mode to drive-control the drive means similarly with passive joints, whereby drive-controlling respective joint portions. The walk controller (30) preferably switches the drive and passive modes with respect to, for example, joint portions of knee and foot portions, or switches to the powered mode for kick-up and landing during walking motion, and to the passive mode for a free foot state.