A negative pressure gauge includes a body, a positioning means, a display model, a power-supplying unit and a switch. The positioning means has a rotary piece pivotally connected to the body. The rotary piece is formed with a trigger protrusion. The display model is fixed to the body. The power-supplying unit is fixed to the body and electrically connected to the display model via an electrical line. The switch is mounted in the body and arranged to correspond to the trigger protrusion of the rotary piece. The switch is mounted on the electrical line. In normal state, the switch generates a broken circuit between the display model and the power-supplying unit. When the rotary piece rotates with respect to the body, the switch will be triggered by the trigger protrusion, so that the power-supplying unit and the display model are electrically connected to each other. Via this arrangement, the display model can be turned on or off selectively to save the electricity.