Water treatment device and water treatment method using same

This water treatment device is provided with a cylindrical body positioned in a substantially vertical direction, uses a treatment section packed into at least one section of the interior of the body to purify a fluid to be treated which is supplied from above, and recovers the treated fluid from below, the water treatment device being further equipped with: a plurality of treatment layers arranged in the axial direction of the interior of the body with intervals interposed therebetween, having particles sealed therein, and having a pair of partition plates for preventing the vertical outflow of the particles therefrom; a bubble supply part for supplying bubbles to the interior of the body from below; a backwash water supply part for supplying backwash water to the interior of the body from below; and a discharge part arranged above each of the treatment layers inside the body for discharging the bubbles and/or the backwash water. Furthermore, there is a space between the particles and the upper partition plate of each treatment layer when in a normal state.