A device for assay can evenly develop solution, and performs highly accurate and sensitive measurement. A first device part (10) maintains a second insoluble carrier (12) and a third insoluble carrier (13) in such a manner that they overlap with each other at a detection portion (14) of a first insoluble carrier (11). These three carriers (11), (12) and (13) are housed not in contact with each other. A pressing unit (18) having a pressing surface (18a) that is parallel to the detection portion (14) is provided on an inner surface of the second device part (20) facing the detection portion (14). The pressing surface (18a) is displaced by being pressed toward the detection portion (14), and presses, from the upper side of the first insoluble carrier (11), the second insoluble carrier (12) and the third insoluble carrier (13) onto the first insoluble carrier (11). The first device part (10) and the second device part (20) are joined together.