Magnetic particle imaging (MPI) is a new imaging modality that is capable of imaging the distribution of magnetic nanoparticles (MNPs) with high sensitivity, high spatial resolution, and high imaging speed. We applied an excitation magnetic field with frequency ω0 and an additional dc bias Hdc to obtain a second-harmonic response of magnetization M generated owing to the nonlinearity of M–H. When Hdc is set at a knee point in the M–H characteristics, the second harmonic reaches a maximum. This method does not need a large ac modulation field Hac. In this study, we developed a two-dimensional (2-D) imaging system using the detection of the second-harmonic response of MNPs. As a result, we successfully obtained the one-dimensional (1-D) image of a phantom 60-nm-diameter nanoparticle in a ϕ2 × 4 mm vessel. In the 1-D imaging map, the full width at half maximum (FWHM) was about 2 mm, which was consistent with the phantom size (ϕ2 mm). In the 20 × 8 mm2 2-D imaging map, the position of the two MNP phantoms could be identified.