To provide a novel method which enables the measurement of the molecular weight distribution of a high-molecular substance.

The pressure P in a container in which the high-molecular substance and a solvent-containing sample are put is changed so as to be raised or lowered to set various pressures and the high-molecular substance present in the solvent phase of the sample is detected under various pressures to measure detection intensity I, and the change rate dI/dP of the detection intensity to the pressure P is calculated and converted to the change rate of the amount of the high-molecular substance to a molecular weight M, for example, dW/d(log M) to calculate the molecular weight distribution of the high-molecular substance. The molecular weight distribution of the high-molecular substance may be calculated using a temperature in place of pressure in this method.