A mixing device capable of mixing a variety of fluids and suitable for miniaturization and low cost operation. In one embodiment, a mixing device includes a housing having an inner space and at least one opening for allowing at least two kinds of fluids to flow into the inner space, at least one pair of nozzle holes passing through one side wall of the housing, and at least one pair of guide portions extending from an outer surface of the housing and protruding up to the respective nozzle holes so that mixed fluids respectively discharged through the at least one pair of nozzle holes collide with each other.