The assay device (10) comprises a micro flow path (76); a porous medium provided near the distal end portion (80) of the micro flow path (76); and a space (82) provided between the micro flow path (76) and the porous medium for controlling the flow rate of a fluid moving from the micro flow path (76) to the space (82). After a fluid moved along the micro flow path (76) based on a lateral flow is brought into contact with the porous medium beyond the space (82) and is absorbed to the porous medium, the fluid is divided by the space (82) so that the fluid stays in the micro flow path (76). With this structure, it is possible to perform solution exchange in the micro flow path without using an external device such as a pump.