The design, development, operation and current performance of MOS (multi-object spectroscopy) mode of MOIRCS is described. MOIRCS (Multi-Object Infrared Camera and Spectrograph) is one of the second-generation instruments for the Subaru Telescope and provides imaging and MOS modes with a 4′ × 7′ field of view for a wavelength range from 0.85 to 2.5 μm. To achieve near-infrared(NIR) MOS up to K-band, MOS mode uses multi-slit masks and a mask exchange system in a cryogenic environment. The masks are housed in a vacuum clewar attached to the MOIRCS main dewar and separated by a large gate valve. The mask clewar is equipped with its own cryogenic cooler and a vacuum pump and is capable of storing eighteen masks. The masks are made of thin aluminum foil. Slits are cut with a laser, with software that corrects for the effects of thermal contraction. The masks are cooled to below 130 K in the mask dewar and transported to the focal plane in the main dewar through the gate valve with a linear motion manipulator. An interlock is equipped on the mask exchange system to secure the cryogenic instrument from accident. Replacing masks can be done in the daytime without breaking the vacuum of the main dewar by isolating the mask dewar with the gate valve. Acquisition occurs by iteratively taking on-sky images through alignment holes on the mask until the rotation and offset between alignment stars and alignment holes become small enough. MOIRCS/MOS mode will be open to the public in late 2006.