A rotary compression mechanism includes: a shaft attached to a casing; a drive cylinder rotatably supported on the shaft; a rotor provided inside the drive cylinder; a transfer mechanism connecting the drive cylinder and the rotor in rotational motion at a constant speed; and a partition plate dividing a space defined between an inner periphery of the drive cylinder and an outer periphery of the rotor. The rotor has a second rotation center which is eccentric with respect to a first rotation center of the drive cylinder such that the outer periphery of the rotor is in contact with the inner periphery of the drive cylinder at a contact portion. The partition plate has a structure by which one end of the partition plate is let in and out in a vicinity of the inner periphery of the drive cylinder or in a vicinity of the outer periphery of the rotor.