A mobile apparatus for moving or acting while avoiding contact with an object, and reducing a change in the behavior of the object. It is determined whether there is a first spatial element that satisfies a contact condition that there is a possibility of contact with a reference spatial element on a discriminant plane including at least part of a representative point trajectory of a robot. If there is a first spatial element that satisfies the contact condition, a second action plan element involving changing the representative point trajectory is assumed, and it is determined whether there is a first spatial element that satisfies the contact condition on a new discriminant plane including at least part of the changed representative point trajectory. If there is no first spatial element that satisfies the contact condition on the new discriminant plane, the assumed new "second action plan element" is set.