A method for detecting a target molecule comprising the steps of: providing a fluid device having a substrate and a well array disposed on the substrate, supplying a dispersion liquid of a structure including a target molecule into the well array of the fluid device, and introducing the structure into wells of the well array, wherein the dispersion liquid of the structure further includes a reagent which is necessary in the step of extracting the target molecule and a step of detecting the target molecule, wherein the dispersion liquid is an aqueous solution; supplying a sealing solution into the well array, forming a layer of the sealing solution on the dispersion liquid introduced in the wells, and sealing the dispersion liquid in the wells, wherein the sealing solution is a liquid immiscible with the dispersion liquid that can individually seal the dispersion liquid introduced in the wells and forms liquid microdroplets of the dispersion liquid; extracting the target molecule from the structure within the sealed wells; and detecting the target molecule within the sealed wells; wherein the method further comprises a step of detecting the structure, within the sealed wells, after the step of introducing the structure into the well and before the step of extracting the target molecule from the structure, after the target substance is extracted from the structure, or simultaneously performed with the step of detecting the target molecule, wherein the structure is at least one of an exosome, cell, bacterium, virus, fungus, and endoplasmic reticulum wherein the extraction and detection steps are performed in the same well. providing a fluid device having a substrate and a well array disposed on the substrate, supplying a dispersion liquid of a structure including a target molecule into the well array of the fluid device, and introducing the structure into wells of the well array, wherein the dispersion liquid of the structure further includes a reagent which is necessary in the step of extracting the target molecule and a step of detecting the target molecule, wherein the dispersion liquid is an aqueous solution; supplying a sealing solution into the well array, forming a layer of the sealing solution on the dispersion liquid introduced in the wells, and sealing the dispersion liquid in the wells, wherein the sealing solution is a liquid immiscible with the dispersion liquid that can individually seal the dispersion liquid introduced in the wells and forms liquid microdroplets of the dispersion liquid; extracting the target molecule from the structure within the sealed wells; and detecting the target molecule within the sealed wells; wherein the method further comprises a step of detecting the structure, within the sealed wells, after the step of introducing the structure into the well and before the step of extracting the target molecule from the structure, after the target substance is extracted from the structure, or simultaneously performed with the step of detecting the target molecule, wherein the structure is at least one of an exosome, cell, bacterium, virus, fungus, and endoplasmic reticulum wherein the extraction and detection steps are performed in the same well.