1. A bottom sediment determining device for being inputted with an echo signal corresponding to an ultrasonic wave outputted underwater, and determining water bottom sediment using a neural network, the device comprising: a memory for storing a set of two or more synaptic connection coefficients to be used in the neural network so as to be associated with positional information; a receiver for receiving the positional information, the positional information representing a location where the water bottom sediment is determined; an acquisition module for acquiring the set of synaptic connection coefficients corresponding to the positional information, said acquiring being accomplished by selecting the set of synaptic connection coefficients from among a plurality of sets of synaptic connection coefficients based on the received positional information, the selected set of synaptic connection coefficients being different from a previously-selected set of synaptic connection coefficients even if a water depth and bottom sediment determination are the same as those associated with the previously-selected set of synaptic connection coefficients; and a setting module for setting the acquired set of synaptic connection coefficients to the neural network.