An opening device including an elongate ejector ram mounted in a longitudinally displaceable manner in a housing and pretensioned at one end by a spring into a position in which it protrudes out of the housing and pushable back into the housing counter to the pretensioning. A releasable locking device locks the ejector ram in a position retracted in the housing. A tensioning device is provided forcibly displaces the ejector ram. When the movable furniture part is completely opened, a tensioning device forcibly displaces the ejector ram into the locking position retracted within the housing. The tensioning device converts the opening movement of the movable furniture part in the interior of the housing via a tensioning member having one end guided out of the housing into a movement pulling the ejector ram back into the retracted position.