The present invention is intended for providing a porous film having excellent film uniformity, and is capable to contribute for improving cyclic and rate properties, which is provided on a surface of electrode used for a secondary battery and the like. The immersion liquid retainer includes a fixing part in which a first penetration hole for viewing an observed portion of a specimen is formed and which is fixed to the specimen, and an immersion liquid retaining part where a concave part that has a bottom surface configured with a transparent flat plate and is intended to retain an immersion liquid is formed. The phase-locked loop is operably coupled to generate a first oscillation from a reference oscillation. If the signals are substantially similar, a thermal asperity is indicated.