Guardrail apparatus for a system scaffold having an orthogonal reference system comprising a width direction, a height direction and a transverse direction, comprising - scaffolding levels (52, 54, 56) having standards arranged spaced apart in the width direction, transoms connected to the standards in the transverse direction and scaffolding decks connected to the transoms so as to run in the width direction, - wherein the guardrail apparatus - is in the form of what is referred to as an advance guardrail apparatus which is mounted from an existing lower scaffolding level for an upper scaffolding level to be mounted thereabove and which then forms a side protection for the upper scaffolding level, - has at least one upper guardrail bar, also called handrail, which runs in the width direction, - has a right-hand vertical guardrail member and a left-hand vertical guardrail member, - the right-hand and left-hand vertical guardrail members (12, 14) have right-hand and left-hand connection units (18.1, 18.2) for releasable connection to the standards of the system scaffold, - characterized in that - a guardrail unit is present, comprising - a right-hand and left-hand inner vertical strut (20, 22), wherein - the upper guardrail bar is connected in the upper end region of the right-hand and left-hand inner vertical strut (20, 22), - the right-hand and left-hand inner vertical struts (20, 22) are respectively connected on the inner side to the right-hand and left-hand vertical guardrail member (12, 14), wherein -- a rotary connection unit for temporarily rotating the guardrail unit about an axis of rotation running in the width direction is present in the lower end region of the right-hand and left-hand vertical strut (20, 22), such that the guardrail unit with its right-hand and left-hand inner vertical strut (20, 22) and with the upper guardrail bar can be pivoted downwards by 180° relative to the right-hand/left-hand vertical guardrail member (12, 14), -- a releasable locking unit for temporarily releasably locking the right-hand and left-hand inner vertical strut (20, 22) to the right-hand and left-hand vertical guardrail member (12, 14) is present in the upper end region of the right-hand and left-hand vertical strut (20, 22). - scaffolding levels (52, 54, 56) having standards arranged spaced apart in the width direction, transoms connected to the standards in the transverse direction and scaffolding decks connected to the transoms so as to run in the width direction, - wherein the guardrail apparatus - is in the form of what is referred to as an advance guardrail apparatus which is mounted from an existing lower scaffolding level for an upper scaffolding level to be mounted thereabove and which then forms a side protection for the upper scaffolding level, - has at least one upper guardrail bar, also called handrail, which runs in the width direction, - has a right-hand vertical guardrail member and a left-hand vertical guardrail member, - the right-hand and left-hand vertical guardrail members (12, 14) have right-hand and left-hand connection units (18.1, 18.2) for releasable connection to the standards of the system scaffold, - characterized in that - a guardrail unit is present, comprising - a right-hand and left-hand inner vertical strut (20, 22), wherein - the upper guardrail bar is connected in the upper end region of the right-hand and left-hand inner vertical strut (20, 22), - the right-hand and left-hand inner vertical struts (20, 22) are respectively connected on the inner side to the right-hand and left-hand vertical guardrail member (12, 14), wherein -- a rotary connection unit for temporarily rotating the guardrail unit about an axis of rotation running in the width direction is present in the lower end region of the right-hand and left-hand vertical strut (20, 22), such that the guardrail unit with its right-hand and left-hand inner vertical strut (20, 22) and with the upper guardrail bar can be pivoted downwards by 180° relative to the right-hand/left-hand vertical guardrail member (12, 14), -- a releasable locking unit for temporarily releasably locking the right-hand and left-hand inner vertical strut (20, 22) to the right-hand and left-hand vertical guardrail member (12, 14) is present in the upper end region of the right-hand and left-hand vertical strut (20, 22). -- a rotary connection unit for temporarily rotating the guardrail unit about an axis of rotation running in the width direction is present in the lower end region of the right-hand and left-hand vertical strut (20, 22), such that the guardrail unit with its right-hand and left-hand inner vertical strut (20, 22) and with the upper guardrail bar can be pivoted downwards by 180° relative to the right-hand/left-hand vertical guardrail member (12, 14), -- a releasable locking unit for temporarily releasably locking the right-hand and left-hand inner vertical strut (20, 22) to the right-hand and left-hand vertical guardrail member (12, 14) is present in the upper end region of the right-hand and left-hand vertical strut (20, 22).