Abstract: Provided is a capsize risk level calculation system which can calculate a capsize risk level providing an index of the capsize risk on an oscillation of hull without using hull information. This system includes an acceleration sensor detecting a reciprocating motion in an up-down direction of a vessel as an oscillation in an up-down direction of a virtual oscillation center axis; an angular velocity sensor detecting a simple pendulum motion in a rolling direction around the vessel center axis as a simple pendulum motion of the vessel COG around the oscillation center axis; and an arithmetic part calculating a capsize risk level from an oscillation radius connecting between the oscillation center axis and the vessel COG, and a capsize limit oscillation radius connecting between the oscillation center axis and the vessel metacenter, which are obtained on the results of detection by the acceleration sensor and the angular velocity sensor.