A flow passage structure can be disposed in a multi-stage centrifugal refrigerant compressor having a deswirl vane and a return channel bend. The flow passage structure includes a two-way flow passage having a first outlet and a second outlet, for diverging externally injected refrigerant; a first side discharge flow passage connected to the first outlet of the two-way flow passage and having a first side outlet disposed below the deswirl vane, such that the diverged refrigerant can be discharged into the return channel bend and uniformly mixed with the refrigerant in the return channel bend; and a second side discharge flow passage connected to the second outlet of the two-way flow passage and having a second side outlet disposed below the deswirl, vane, such that the diverged refrigerant can be injected into the return channel bend and uniformly mixed with the refrigerant in the return channel bend.