A cap used when priming a liquid flow path includes: a case body (10) having a connection hole (12a) to be connected to a connection part (31) provided on the liquid flow path, a liquid storing chamber (21) continuing from the connection hole (12a), and a discharge opening (13) continuing from the liquid storing chamber (21) and a filter (22) formed from a gas-permeable and liquid-impermeable material and is attached to the case body (10) so as to cover the discharge opening (13). The cap is characterized by comprising a seal member (20, 40, 50) which is attached to the case body (10) so as to close the connection hole (12a) and configured in such a manner that the liquid flow path can communicate with the liquid storing chamber (21) when the connection part (31) is connected to the connection hole (12a).
