1. A robot system configured to successively convey a plurality of stacked objects to a target site, the robot system comprising: a three-dimensional sensor configured to acquire three-dimensional information of the objects; a robot which comprises a gripping device capable of gripping at least one of the objects; a first three-dimensional information acquiring part configured to acquire three-dimensional information of the stacked objects by the three-dimensional sensor as first three-dimensional information; an object position acquiring part configured to identify a conveyance object which is an object to be conveyed among the objects and acquire the position of the conveyance object, based on the first three-dimensional information; a second three-dimensional information acquiring part configured to acquire, after the conveyance object is gripped by the gripping device and is moved away from other remaining objects by the robot, three-dimensional information of the remaining objects by the three-dimensional sensor as second three-dimensional information; a shape information acquiring part configured to acquire, based on the first three-dimensional information and the second three-dimensional information, three-dimensional shape information of the conveyance object which includes a part which is positioned at a dead angle of the three-dimensional sensor when the first three-dimensional information is acquired; a placement information acquiring part configured to acquire, based on the three-dimensional shape information of the conveyance object which is acquired by the shape information acquiring part, at least one of a distance from a contact part of the conveyance object to a reference point of the gripping device or the robot and a stable posture of the conveyance object, the contact part being a part of the conveyance object on which the conveyance object contacts the target site when the conveyance object is placed at the target site, the stable posture being a posture with which the conveyance object can be stably placed at the target site; and a position and posture calculating part configured to calculate a position and posture of the robot when the conveyance object is placed at the target site, based on at least one of the distance from the contact part to the reference point and the stable posture.