A method for real-time measurement of ultra-high vacuum (UHV) or lower vacuum has been developed by field emission microscopy with tungsten and rhenium tips. Two different methods of pressure measurement were examined: (1) In the first method, the pressure P is determined with the equation P α d log I/d t under a constant applied voltage, where I is the emission current. (2) In the second method, the pressure is determined from P α (d V/d t)/ V 2 with a constant emission current, where V is the applied voltage to keep the emission current constant.