An observation sample is prepared by immobilizing a nanomaterial on a substrate 10 by applying a voltage between a nanomaterial dispersion liquid 13, filled in an interior of an electrostatic spray nozzle 20, and the observation substrate 10 to electrostatically spray and dry the dispersion liquid 13 and electrostatically deposit the nanomaterial. With respect to the observation substrate 10, including a conductive grid portion 11 and a supporting film 12, a reference electrode 81 is disposed below the substrate 10 and a bias voltage of the same polarity as the electrostatic spraying voltage is applied to the grid portion 11 of the substrate 10 to adjust immobilization positions of the nanomaterial on the substrate 10. An observation sample, with which the nanomaterial is immobilized in a satisfactory state on the substrate, can thereby be prepared.