An object of the present disclosure is to enable behavior of a linear structure in water to be estimated. The present disclosure is an apparatus that: expresses at least one parameter in a plurality of discrete values, and forecasts behavior of a linear structure by using each of the discrete values; acquires behavior of the linear structure detected using an optical fiber; calculates a likelihood between the forecasted behavior of the linear structure and the acquired behavior of the linear structure; updates at least one of the plurality of discrete values on the basis of the likelihood; and calculates behavior of the linear structure by using the updated plurality of discrete values.