The present invention relates to an implantable device for improving the pump function of the heart of a human patient by applying an external force on the heart muscle, said device comprising at least one pump device comprising: a first part having a first surface, and a second part having a second surface. The first part is displaceable in relation to the second part and said first and second surfaces abut each other, at least partially. The second part exerts, directly or indirectly, force on an external part of said heart muscle.