A porosity measuring device comprising: a magnetic field generating section configured to generate a magnetic field gradient to cause movement of a dispersoid (s) according to the magnetic field gradient, the dispersoid (s) being dispersed in a dispersion medium (m) flowing in a tubular member; and a dispersoid measuring section configured to measure the movement of the dispersoid (s) dispersed in the dispersion medium (m) in a state where the magnetic field gradient is generated by the magnetic field generating section, characterized by an operating section configured to obtain a porosity P of the dispersoid (s) on the basis of a measurement result of the dispersoid measuring section, wherein the operating section obtains a magnetophoretic velocity v of the dispersoid (s) from the measurement result of the dispersoid measuring section, obtains a volume susceptibility X s of the dispersoid (s) from the magnetophoretic velocity v of the dispersoid (s), and obtains the porosity P, expressed as P = V p /V s of the dispersoid (s) on the basis of the volume susceptibility X s of the dispersoid (s), a volume relationship of the dispersoid (s) that may be defined as V s = V b + V p, and a relationship in magnetization of the dispersoid (s) that may be defined as X s V s = X b V b + X p V p, wherein V s is a volume of the dispersoid s, V b is a volume of the main body part b of the dispersoid s, V p is a volume of the void part p of the dispersoid s, X b is the volume susceptibility of the main body part b of the dispersoid s, and X p is the volume susceptibility of the void part p of the dispersoid s. a magnetic field generating section configured to generate a magnetic field gradient to cause movement of a dispersoid (s) according to the magnetic field gradient, the dispersoid (s) being dispersed in a dispersion medium (m) flowing in a tubular member; and a dispersoid measuring section configured to measure the movement of the dispersoid (s) dispersed in the dispersion medium (m) in a state where the magnetic field gradient is generated by the magnetic field generating section, characterized by an operating section configured to obtain a porosity P of the dispersoid (s) on the basis of a measurement result of the dispersoid measuring section, wherein the operating section obtains a magnetophoretic velocity v of the dispersoid (s) from the measurement result of the dispersoid measuring section, obtains a volume susceptibility X s of the dispersoid (s) from the magnetophoretic velocity v of the dispersoid (s), and obtains the porosity P, expressed as P = V p /V s of the dispersoid (s) on the basis of the volume susceptibility X s of the dispersoid (s), a volume relationship of the dispersoid (s) that may be defined as V s = V b + V p, and a relationship in magnetization of the dispersoid (s) that may be defined as X s V s = X b V b + X p V p, wherein V s is a volume of the dispersoid s, V b is a volume of the main body part b of the dispersoid s, V p is a volume of the void part p of the dispersoid s, X b is the volume susceptibility of the main body part b of the dispersoid s, and X p is the volume susceptibility of the void part p of the dispersoid s.