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We developed a highly sensitive measurement system of ac magnetic fields for magnetic nanoparticle imaging (MPI). First, we developed a detection system using pickup coils made of cooled Cu Litz-wire and high-critical-temperature superconductor (HTS) tape. The pickup coils were connected to a resonant capacitor in order to enhance the signal voltage generated in the pickup coils. The magnetic field noise at the resonant frequency of about 9 kHz was as low as 13 fT/Hz1/2 and 12 fT/Hz1/2 for the Cu and HTS coils, respectively. Next, we demonstrated the detection of nanoparticles using the cooled Cu coil and the third harmonic signal generated by the nonlinear magnetization of nanoparticles. An excitation field having a frequency of 3 kHz and root mean square value of 1.6 mT was applied to the magnetic particles, and the third harmonic signal at 9 kHz was detected that reduced the interference from the excitation field. We demonstrated the detection of 100 μg of magnetic nanoparticles. We obtained a clear contour map of the magnetic field from the particles, and could detect the particles located as far as 100 mm under the pickup coil.