Monitoring device for a water supply network, which is installed on a hydrant a) the hydrant having: aa) a riser pipe which is intended to be connected to the water supply network and on which an optional riser pipe extension can be mounted; ab) a main valve to interact with a valve seat; ac) a valve rod which is connected to the main valve and on which an optional valve rod extension can be mounted; ad) a spindle nut which is connected to the valve rod or the valve rod extension and in which a spindle engages; and ae) a spindle bearing, on which the spindle is supported axially and which can be locked in the riser pipe or in the riser pipe extension; and b) the monitoring device having a signal unit, which has: ba) at least one detector unit (46,49) to detect the status in the water supply network and/or on the hydrant; bb) a battery serving to supply power to the signal unit; and bc) a transmitting antenna for transmitting the signals detected by the at least one detector unit (46,49), characterized in that c) the signal unit comprises a base part which is mounted on the riser pipe or on the riser pipe extension and which has an axial passage and a holding compartment accessible from outside to accommodate the battery. a) the hydrant having: aa) a riser pipe which is intended to be connected to the water supply network and on which an optional riser pipe extension can be mounted; ab) a main valve to interact with a valve seat; ac) a valve rod which is connected to the main valve and on which an optional valve rod extension can be mounted; ad) a spindle nut which is connected to the valve rod or the valve rod extension and in which a spindle engages; and ae) a spindle bearing, on which the spindle is supported axially and which can be locked in the riser pipe or in the riser pipe extension; and aa) a riser pipe which is intended to be connected to the water supply network and on which an optional riser pipe extension can be mounted; ab) a main valve to interact with a valve seat; ac) a valve rod which is connected to the main valve and on which an optional valve rod extension can be mounted; ad) a spindle nut which is connected to the valve rod or the valve rod extension and in which a spindle engages; and ae) a spindle bearing, on which the spindle is supported axially and which can be locked in the riser pipe or in the riser pipe extension; and b) the monitoring device having a signal unit, which has: ba) at least one detector unit (46,49) to detect the status in the water supply network and/or on the hydrant; bb) a battery serving to supply power to the signal unit; and bc) a transmitting antenna for transmitting the signals detected by the at least one detector unit (46,49), characterized in that ba) at least one detector unit (46,49) to detect the status in the water supply network and/or on the hydrant; bb) a battery serving to supply power to the signal unit; and bc) a transmitting antenna for transmitting the signals detected by the at least one detector unit (46,49), characterized in that c) the signal unit comprises a base part which is mounted on the riser pipe or on the riser pipe extension and which has an axial passage and a holding compartment accessible from outside to accommodate the battery.