A flow passage device is provided, with which a liquid is easily introduced and introduction of air bubbles into a flow passage along with a liquid when the liquid is introduced into the flow passage can be decreased.A flow passage device according to the present invention includes a flow passage through which a fluid flows in a direction different from a vertical direction, an introducing space that has a supply port opening toward the vertical direction and introduces a liquid into the flow passage, and a buffering space that connects the flow passage to the introducing space. The buffering space allows a gas-liquid interface to be formed due to surface tension. A direction in which the gas-liquid interface faces is different from the vertical direction.