A brake master cylinder is disclosed. A brake master cylinder comprises a cylinder body having a hollow, a piston provided to reciprocate in the hollow, and a sensor module connected on the cylinder body and configured to sense a movement according to a reciprocating motion of the piston. And the sensor module includes a sensor housing coupled to a guide hole formed parallel with the hollow of the cylinder body and having a hollow therein in a length direction, a magnet installed to be slidable through the hollow of the sensor housing, and an actuating member coupled to the piston and configured to move the magnet according to a movement of the piston.