A water solution (20) in which an observation sample (10) is, for example, dissolved is sandwiched on a first insulative thin film (203) side provided under a conductive thin film 201. When an electron beam incident part is charged minus, electric dipoles of water molecules are arrayed along a potential gradient. Electric charges are also generated on the surface of a second insulative thin film (204). The electric charges are detected by a terminal section (210) and changes to a measurement signal. In a state in which an electron beam (102) is blocked, the minus potential disappears. Consequently, the electric charges on the surface of the first insulative thin film (203) also disappear, and the measurement signal output from the terminal section (210) changes to 0.