According to an embodiment, a detection device is used for a structure. The structure includes: a first member; a second member provided on an opposite side of the first member; and a welded portion that is provided along an end of the second member facing the first member and fixes the first member and the second member. The detection device include: a plurality of acoustic emission sensors that are disposed so as to be spaced apart from each other in a direction in which the welded portion extends and are configured to detect an elastic wave transmitted to the second member, each acoustic emission sensor being attached to the second member; and an outputter that outputs information, obtained from outputs of the plurality of the acoustic emission sensors.