A device for assay, the device comprising: a complex part including a first device part and a second device part that are joined together in a vertical direction, wherein the first device part includes a first insoluble carrier housed therein, the first insoluble carrier having a detection portion containing a substance that specifically binds to an analyte, and wherein the second device part has a hole for injecting sample solution into the first device part, and wherein a second insoluble carrier for developing solution and a third insoluble carrier for absorbing solution are housed in the complex part, and wherein the second insoluble carrier and the third insoluble carrier are kept in such a manner that the second insoluble carrier and the third insoluble carrier overlap with each other at the detection portion of the first insoluble carrier, and wherein the first insoluble carrier, the second insoluble carrier and the third insoluble carrier are housed in such a manner that they are not in contact with each other, and wherein a pressing unit having a pressing surface that faces the detection portion of the first insoluble carrier is provided in the second device part that faces the detection portion of the first insoluble carrier, and wherein the pressing surface is displaced by being pressed toward the detection portion, and presses, from the upper side of the first insoluble carrier, the second insoluble carrier and the third insoluble carrier onto the first insoluble carrier, and wherein a rib that regulates the displacement of the pressing surface by abutting a part of the pressing unit is provided on an inner surface of the first device part, the inner surface facing the pressing unit. a complex part including a first device part and a second device part that are joined together in a vertical direction, wherein the first device part includes a first insoluble carrier housed therein, the first insoluble carrier having a detection portion containing a substance that specifically binds to an analyte, and wherein the second device part has a hole for injecting sample solution into the first device part, and wherein a second insoluble carrier for developing solution and a third insoluble carrier for absorbing solution are housed in the complex part, and wherein the second insoluble carrier and the third insoluble carrier are kept in such a manner that the second insoluble carrier and the third insoluble carrier overlap with each other at the detection portion of the first insoluble carrier, and wherein the first insoluble carrier, the second insoluble carrier and the third insoluble carrier are housed in such a manner that they are not in contact with each other, and wherein a pressing unit having a pressing surface that faces the detection portion of the first insoluble carrier is provided in the second device part that faces the detection portion of the first insoluble carrier, and wherein the pressing surface is displaced by being pressed toward the detection portion, and presses, from the upper side of the first insoluble carrier, the second insoluble carrier and the third insoluble carrier onto the first insoluble carrier, and wherein a rib that regulates the displacement of the pressing surface by abutting a part of the pressing unit is provided on an inner surface of the first device part, the inner surface facing the pressing unit.