The invention is objected to enable the provision of an vessel steel plate measuring device capable of measuring the thickness of vessel steel plate, even in the case of a vessel having a vessel mirror part formed by a spherical or conical curved surface such as a reactor (reaction vessel) and various kinds of obstructions at a vessel barrel. The structure of the device is characterized in that a supporting point member 20 is detachably provided from a supporting point set at the central axis of the vessel mirror part 1a of a reactor 1, and a traveling carrier 6 traveling on a steel plate of the vessel mirror part 1a and having plural ultrasonic probes 7 mounted thereon is connected to one end of a turning radius regulating member 19 rotatively provided around the supporting point member 20.