A power assist robot apparatus is disclosed which is capable of assisting heavy-object lifting action and walking movement with fewer driving sources, and a method is disclosed for controlling the power assist robot apparatus. Two power-assist electric motors are located near opposite lateral sides in a right-left direction of the wearer's waist, respectively. Each lower-limb assist arm has one end fixed to a rotary shaft of the power-assist electric motor and the other end to which a lateral side of the thigh is attached. An upper-body assist arm placed at the wearer's chest and a main frame that holds the two power-assist electric motors at both ends thereof and is placed at the wearer's waist are connected by a driven rotary shaft which is rotatable about a vertical axis and a driven rotary shaft which is rotatable about a right-left axis.