We developed a magnetorelaxation (MRX) measurement system using magneto-resistive (MR) sensors in order to evaluate the core size of magnetic nanoparticles (MNPs). The MRX signal can be measured from 0.1 to 10^4 s. In this study, the Néel relaxation time of immobilized Resovist, SHP25, and SHP30 MNPs were measured. The result shows that the ascending order of the Néel relaxation time is with SHP25, Resovist, and SHP30, respectively.