According to one embodiment, a detection system includes a plurality of sensors, a locator, a first counter, and a determiner. The plurality of sensors that detect an elastic wave, the sensors being disposed separately from each other in a direction in which the welded portion extends and each being installed on the first member or the second member. The locator that locates a generation source position of the elastic wave on the basis of outputs of the plurality of sensors. The first counter that accumulates information of generation source positions of a plurality of elastic waves located by the locator to calculate a distribution of generation source positions of the plurality of elastic waves over a predetermined time. The determiner that determines the position of the crack on the basis of the distribution calculated by the first counter.