An in vivo potential measurement device includes an insulating member and an amplifier. The insulating member has an electrode. The insulating member is inserted into an organ of a living body such that an outer peripheral face of the insulating member contacts with an inner wall face of the organ at a contact site. The electrode senses electric potential at the contact site. The amplifier amplifies the electric potential to obtain output voltage. The amplifier has input capacitance Cin and input resistance Rin that satisfy Cin/Ce>；0.1 and 1/(2πfCeRin)>；0.1, where Ce represents capacitance of the insulating member at the contact site, and f represents frequency of the electric potential at the contact site on the inner wall face. A contact state between the outer peripheral face and the inner wall face is evaluated using the output voltage.
