light emitted from the light source 4 according to the output adjusting signal from the comparator 64 therefore the sum of the photo electric currents ia and ib is always maintained equal to the constant reference value therefore ia ib is constant substantially irrespective of the directionality of the light source directional change of the incoming light 42 due to the slit position change and the change in the distance between the light source and the light impinging position the barometric meter of the present invention further comprises a display unit 7 the display unit 7 comprises a voltage converter 71 connected at its input 72 to the first current to voltage converter 61 and a digital display 74 connected to an output 73 of the voltage converter 71 thus the amount ia proportional to the displacement x of the light impinging position is taken out as a voltage vi at the output of the current voltage converter 61 this voltage is converted by the voltage converter 71 of the display 7 into a voltage vo of a numerical value which is equal to the numerical value of the air pressure or altitude and read as a voltage by a digital display 74 similar to a ordinary digital display voltmeter and displayed as the pressure or altitude fig5 illustates the detail of the voltage converter 71 shown in fig4 the voltage converter 71 comprises an amplifier 711 connected to the output of the current to voltage converter 61 fig4 the output of the amplifier.