A filtration device includes a container, a channel member, and a filter. The container has a space of variable volume that contains a liquid, the liquid including a target substance to be separated by filtration. The channel member includes a channel and an opening that are defined therein, the opening being provided at a position along the channel in a direction transverse to the direction in which the channel extends. The channel member has an attachment part to which the container is attached and which provides communication between the space in the container and the channel. The filter is attached inside the channel member and positioned at the opening.