A wearable assist robot apparatus (1, 201, 301, 401, 401a, 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 (1 1)and the respective left and right thighs of the wearer about the axial line in the lateral direction; an acceleration sensor provided on the harness device, the acceleration sensor being configured to detect an acceleration of the body trunk; landing determination means configured to determine a foot landing state in response to an output from the acceleration sensor; 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 a supporting leg in a landed state 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 a free leg not in contact with the land by a free-leg side drive source of the pair of drive sources, wherein, based on a local maximum value of acceleration in a vertical direction of the body trunk which value is detected by the acceleration sensor, the landing determination means determines that one of the feet lands, and the landing determination means compares 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 at a landing time, and determines the leg which shows a larger angle to be the landed supporting leg and the leg which shows a smaller angle is determined to be the free leg. 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 (1 1)and the respective left and right thighs of the wearer about the axial line in the lateral direction; an acceleration sensor provided on the harness device, the acceleration sensor being configured to detect an acceleration of the body trunk; landing determination means configured to determine a foot landing state in response to an output from the acceleration sensor; 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 a supporting leg in a landed state 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 a free leg not in contact with the land by a free-leg side drive source of the pair of drive sources, wherein, based on a local maximum value of acceleration in a vertical direction of the body trunk which value is detected by the acceleration sensor, the landing determination means determines that one of the feet lands, and the landing determination means compares 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 at a landing time, and determines the leg which shows a larger angle to be the landed supporting leg and the leg which shows a smaller angle is determined to be the free leg.