A fluid passage closure system in a compressor includes a fluid passage, a seal cap unit and a flow space. The fluid passage formed in the housing of the compressor includes a large-diameter and the small-diameter passages, a step formed between the large-diameter and the small-diameter passages and a flat portion formed in the step. The large-diameter passage is formed on the opening side of the fluid passage and connected to the small-diameter passage. The seal cap unit closing the fluid passage includes a flat sealing surface and a press-contact surface. The flat sealing surface is in contact with the flat portion to close the fluid passage. The press-contact surface is formed to be pressed against the peripheral wall surface so that the seal cap unit is kept in the fluid passage. The flow space is formed between the press-contact surface and the peripheral wall surface.