A method of estimating the number of microparticles in a sample, comprising: a constant flow counting step of counting by constant flow the number of the target microparticles contained in the sample at a predetermined flow rate; a sectioning step of sectioning measurement data obtained as a result of the constant flow counting, indicating a relationship between elapsed time and the number of detected microparticles, into a predetermined number of sections by a predetermined unit time for a section; a section number counting step of counting the number of sections in which the microparticles are detected and the number of sections in which the microparticles are not detected, in the predetermined number of sections; and a concentration calculating step of estimating the number of the microparticles in the sample, by a statistical method from the flow rate of the sample in the constant flow counting step, the predetermined number of sections and the predetermined unit time in the sectioning step, and the number of sections in which the microparticles are detected in the section number counting step; characterized in that the statistical method is a most probable number method; and in the sectioning step, the measurement data are sectioned into two or more predetermined number of rows of sections by two or more predetermined unit times, and in the concentration calculating step, the number of the microparticles in the sample is estimated, by a most probable number method from the flow rate of the sample in the constant flow counting step, the predetermined number of sections and the two or more predetermined unit times in the sectioning step, and the number of sections according to each of the unit times in which the microparticles are detected in the section number counting step. a constant flow counting step of counting by constant flow the number of the target microparticles contained in the sample at a predetermined flow rate; a sectioning step of sectioning measurement data obtained as a result of the constant flow counting, indicating a relationship between elapsed time and the number of detected microparticles, into a predetermined number of sections by a predetermined unit time for a section; a section number counting step of counting the number of sections in which the microparticles are detected and the number of sections in which the microparticles are not detected, in the predetermined number of sections; and a concentration calculating step of estimating the number of the microparticles in the sample, by a statistical method from the flow rate of the sample in the constant flow counting step, the predetermined number of sections and the predetermined unit time in the sectioning step, and the number of sections in which the microparticles are detected in the section number counting step; characterized in that the statistical method is a most probable number method; and in the sectioning step, the measurement data are sectioned into two or more predetermined number of rows of sections by two or more predetermined unit times, and in the concentration calculating step, the number of the microparticles in the sample is estimated, by a most probable number method from the flow rate of the sample in the constant flow counting step, the predetermined number of sections and the two or more predetermined unit times in the sectioning step, and the number of sections according to each of the unit times in which the microparticles are detected in the section number counting step. the statistical method is a most probable number method; and in the sectioning step, the measurement data are sectioned into two or more predetermined number of rows of sections by two or more predetermined unit times, and in the concentration calculating step, the number of the microparticles in the sample is estimated, by a most probable number method from the flow rate of the sample in the constant flow counting step, the predetermined number of sections and the two or more predetermined unit times in the sectioning step, and the number of sections according to each of the unit times in which the microparticles are detected in the section number counting step.