An automatic locking valve for safely administering a dose of medication is disclosed. The automatic locking valve comprises a valve housing defined with a space and formed with input and output ports for intaking and discharging the medication, a spool-shaped on/off member having a diameter smallest at the center thereof and being tapered to linearly increase toward upper and lower portions of the on/off member so that the on/off member moves up and down depending on a closed state of the door, and a membrane attached to the space of the valve housing such that the membrane expands toward a lower portion of the space due to an elasticity of the membrane when the tube is blocked. The medication can be safely and accurately administered under a condition of installation with the door closed above the tube.