A crystallization device is for protein crystallization with a small amount of a sample in the liquid to liquid diffusion method. It is easy to fill the device with protein solution and precipitant solution and easy to pick up grown crystals from the device. The device comprises a channel plate made of polydimethylsiloxane (PDMS) and the first and second cover sheets made of polyethylene terephthalate. The channel plate includes at least one elongated channel having one side which extends in the longitudinal direction of the channel, the one side being exposed at the bottom surface of the channel plate. The channel has both ends which communicate with a protein solution inlet and a precipitant solution inlet respectively. The channel also communicates midway with a gel inlet and a vent hole. When picking up grown crystals from the device, the second cover sheet is cut off with a cutter knife so that the channel is exposed.