The physical quantity measurement device involves a terminal unit, a sensor, a storage unit, an output unit, a communication unit, a control unit, and a discriminating unit. The terminal unit has a power terminal, an output terminal, and a ground terminal. The communication unit uses the power terminal in order to receive a serial signal from an external device. The control unit has an adjusting mode and a normal mode. The discriminating unit decides that the external device requests either the adjusting mode or the normal mode based on whether or not an electrical potential of each of the power terminal and the output terminal satisfies a predetermined condition. The control unit selects the normal mode when the discriminating unit decides that the external device requests the normal mode, and selects the adjusting mode when the discriminating unit decides that the external device request the adjusting mode.