In a penstock internal maintenance system, the penstock includes an inclined portion between upper and lower ends, the upper end arranged in an edifice including a water collecting chamber and a gate bearing structure allowing a gate to close the penstock, the gate bearing structure adjacent the water collecting chamber forming a vertical pit. The system includes a set of units assembled in an assembled configuration and separated in a dismounted configuration, the units including an anchor unit, a launching unit and a penstock inspection platform unit, the units in the dismounted configuration being enter the penstock through the gate bearing structure adjacent the water collecting chamber, the units are assembled when located in the penstock, the anchor unit slidingly received and retained in the gate bearing structure, the anchor unit having two lateral edges being guided and retained in two vertical lateral grooves of the gate bearing structure.