It is an object of the present teachings to provide a technique for effectively providing a packed region in a water treatment apparatus in which the packed region is provided in a path of water to be treated and is filled with biomembrane-deposited members. According to the present invention, a representative water treatment apparatus may include a packed region provided in a path of water to be treated, a first packed part arranged in the packed region, a block of first biomembrane-deposited members that are packed in the first packed part, a second packed part that is arranged under the first packed part, and a plurality of second biomembrane-deposited members that are packed in the second packed part. According to the invention, the first biomembrane-deposited members are configured to prevent the second biomembrane-deposited members from moving into the first packed part by pressing the second biomembrane-deposited members from above. Therefore, the packed region can be formed in simpler structure without adopting any specific devices exclusively designed to keep the second biomembrane-deposited members within the second packed part.