A process for producing a mass transfer device is provided which enables packing of a packing material uniformly in a flow channel, and transfers a specified substance by flowing a fluid containing the specified substance through a flow channel on a substrate. A mass transfer device produced by the process is also provided. The process for producing a mass transfer device comprises steps of preparing a substrate, forming a flow channel on the surface of the substrate, applying a liquid drop composed of a packing material and a liquid medium, and packing the packing material in the flow channel by removing the liquid medium.