A thickness-determining apparatus for determining a wall thickness of a microcapsule having a refractive index n1 comprises a sensor 1 for detecting a light scattering intensity data I1 with respect to the microcapsule dispersed in a medium having a refractive index n1, and a light scattering intensity data I2 with respect to the microcapsule dispersed in a medium having a refractive index n2; a memory circuit 3 for storing a theoretical equation correlating a light scattering intensity characteristic with a particle size; and an arithmetic circuit 5 for calculating an inner diameter r1 and an outer diameter r2 from the theoretical equation based on the light scattering intensity data I1 and I2, and calculating the wall thickness.