The present invention relates to a device for producing a hardenable mass, preferably bone substitute and/or bone reinforcing material or bone cement or similar material. A mixing container (3) has a mixing space (4) in which at least one powder and at least one liquid component (5, 6) are mixed to provide the hardenable mass (2). A piston means (7) is provided in the mixing space (4) of the mixing container (3). At least one means (8) which is rotatable relative to the mixing container (3) cooperates with the piston means (7) for, in a retaining position (P1), retaining said piston means (7) relative to the mixing container (3) and, by rotation to a release position (P2), releasing the piston means (7) such that said piston means can move in a direction (U) towards at least one opening (49) through which said mass (2) can pass out of the mixing space (4). The rotatable means (8) is provided such that it in its release position (P2) can follow the piston means (7) in the mixing space (4) in the direction (U) towards said opening (49).