A wearable assist robot apparatus (1, 201, 301, 401, 401a, 431 501, 551, 601, 651, 701, 751, 801, 851), comprising: a harness device to be worn and retained by a wearer; an assist drive mechanism provided on the harness device, including a pair of drive sources disposed respectively on both left and right sides of a lower body trunk of the wearer configured to generate drive torque about an axial line in a lateral direction of the wearer, the assist drive mechanism being configured to apply assisting force moments respectively to portions of the wearer between a body trunk and respective left and right thighs by drive torque from the respective drive sources; a pair of angle sensors (67,67L,67R) configured to detect relative angles between the body trunk and the respective left and right thighs of the wearer about the axial line in the lateral direction; landing determination means configured to determine a foot landing state in response to an output from the angle sensors (67, 67L, 67R) and determine which leg is a supporting leg in a landed state and which leg is a free leg not in contact with the land; and drive control means configured to, in response to outputs from the respective angle sensors (67,67L,67R) and the landing determination means, apply an assisting force moment in a direction of support to the supporting leg by a supporting-leg side drive source of the pair of drive sources, and apply a swing-up force moment in a swing-up direction to the free leg by a free-leg side drive source of the pair of drive sources, wherein after the assisting force moment applied to the supporting leg is maintained at a predetermined certain value for a first predetermined time (W c), the assisting force moment applied to the supporting leg is reduced in proportion to the angle of the supporting leg detected by the respective angle sensor until the angle of the supporting leg reaches a predetermined value or the free leg is grounded, and wherein after the swing-up force moment applied to the free leg is maintained at a predetermined certain value for a second predetermined time (W d), the swing-up force moment is reduced at a constant predetermined speed until the angle of the free leg reaches a predetermined value, and the landing determination means is configured to determine the foot landing state by comparing the values of the angle (θR) detected by the angle sensor (67,67R) for the right foot and the angle (θL) detected by the angle sensor (67,67L) for the left foot. a harness device to be worn and retained by a wearer; an assist drive mechanism provided on the harness device, including a pair of drive sources disposed respectively on both left and right sides of a lower body trunk of the wearer configured to generate drive torque about an axial line in a lateral direction of the wearer, the assist drive mechanism being configured to apply assisting force moments respectively to portions of the wearer between a body trunk and respective left and right thighs by drive torque from the respective drive sources; a pair of angle sensors (67,67L,67R) configured to detect relative angles between the body trunk and the respective left and right thighs of the wearer about the axial line in the lateral direction; landing determination means configured to determine a foot landing state in response to an output from the angle sensors (67, 67L, 67R) and determine which leg is a supporting leg in a landed state and which leg is a free leg not in contact with the land; and drive control means configured to, in response to outputs from the respective angle sensors (67,67L,67R) and the landing determination means, apply an assisting force moment in a direction of support to the supporting leg by a supporting-leg side drive source of the pair of drive sources, and apply a swing-up force moment in a swing-up direction to the free leg by a free-leg side drive source of the pair of drive sources, wherein after the assisting force moment applied to the supporting leg is maintained at a predetermined certain value for a first predetermined time (W c), the assisting force moment applied to the supporting leg is reduced in proportion to the angle of the supporting leg detected by the respective angle sensor until the angle of the supporting leg reaches a predetermined value or the free leg is grounded, and wherein after the swing-up force moment applied to the free leg is maintained at a predetermined certain value for a second predetermined time (W d), the swing-up force moment is reduced at a constant predetermined speed until the angle of the free leg reaches a predetermined value, and the landing determination means is configured to determine the foot landing state by comparing the values of the angle (θR) detected by the angle sensor (67,67R) for the right foot and the angle (θL) detected by the angle sensor (67,67L) for the left foot.