Void fraction measurement method

The present invention measures the void fraction of an oil by means of a simple method. This method comprises: a step for obtaining, from the relationship of the product of the pressure and the amount of volume change of a plurality of sample oils with known void fractions that have been introduced into an enclosed space relative to the pressure when the plurality of sample oils have been compressed to a predetermined pressure, a first-order line which connects the values of the product of the pressure and the amount of volume change at 0 kPa for each of the sample oils, and establishing said first-order line as a calibration line; a step for obtaining, from the relationship of the product of the pressure and the amount of volume change of a sample oil with an unknown void fraction relative to the pressure when the sample oil is compressed to the predetermined pressure, the value of the product of the pressure and the amount of volume change at 0 kPa for the unknown sample oil; and a step for comparing said value to the calibration line to obtain the void fraction of the unknown oil sample.