A portable cassette holder includes a frame detachably coupled to an outer surface of an automated teller machine having a housing medium entrance, a conveying roller unit and a power supply connection unit. The frame includes a frame medium entrance formed at a position corresponding to the housing medium entrance and configured to mount a cassette thereon. The conveying roller unit is coupled to the frame at a position at which the frame medium entrance is provided, the conveying roller unit configured to convey a medium passing through the frame medium entrance. The power supply connection unit is provided in the frame and configured to be electrically connected to the automated teller machine so as to receive electric power.