An apparatus for setting a penetration depth of a tubular or rod-shaped sliding part, in particular of a medical aspiration needle carrier part in a receiving part, with a handle piece, which surrounds the sliding part, and which is formed for sliding along a longitudinal axis of the sliding part in relation to the sliding part, and the handle piece comprising: - an adjusting element, which is formed for rotating transversely to the longitudinal axis of the sliding part, and - at least one rocker element, which is arranged between the adjusting element and the sliding part, wherein - the adjusting element is formed to pivot the respective rocker element into an arresting pose to the sliding part, in which sliding of the handle piece along the longitudinal axis is blocked, by rotating into a blocking direction transversely to the longitudinal axis into a blocking position, and - the adjusting element is formed to pivot the respective rocker element into a release pose to the sliding part, in which sliding of the handle piece along the longitudinal axis is released, by rotating into an adjusting direction different from the blocking direction transversely to the longitudinal axis into an adjusting position different from the blocking position, characterized by - at least one fixing device with a first fixing element rotatable transversely to the longitudinal axis to the sliding part by means of the adjusting element and a second fixing element stationary to the sliding part against the rotation transverse to the longitudinal axis, wherein - the first and the second fixing element (16a, 16b) are formed to exert a retaining force for fixing the adjusting element against the rotation transverse to the longitudinal axis in the blocking position, wherein - the first fixing element is included by the adjusting element and the retaining force has to be overcome for releasing the fixing. - an adjusting element, which is formed for rotating transversely to the longitudinal axis of the sliding part, and - at least one rocker element, which is arranged between the adjusting element and the sliding part, wherein - the adjusting element is formed to pivot the respective rocker element into an arresting pose to the sliding part, in which sliding of the handle piece along the longitudinal axis is blocked, by rotating into a blocking direction transversely to the longitudinal axis into a blocking position, and - the adjusting element is formed to pivot the respective rocker element into a release pose to the sliding part, in which sliding of the handle piece along the longitudinal axis is released, by rotating into an adjusting direction different from the blocking direction transversely to the longitudinal axis into an adjusting position different from the blocking position, characterized by - at least one fixing device with a first fixing element rotatable transversely to the longitudinal axis to the sliding part by means of the adjusting element and a second fixing element stationary to the sliding part against the rotation transverse to the longitudinal axis, wherein - the first and the second fixing element (16a, 16b) are formed to exert a retaining force for fixing the adjusting element against the rotation transverse to the longitudinal axis in the blocking position, wherein - the first fixing element is included by the adjusting element and the retaining force has to be overcome for releasing the fixing.