The present invention precisely measures characteristic values (such as the absorption coefficient) of an electromagnetic wave when a density of a PM in a DPF which collects the PM in an exhaust gas. The DPF receives the exhaust gas, and collects the PM in the exhaust gas. The DPF includes first hole portions which are open at a first end on a side for receiving the exhaust gas, and are closed at a second end on a side opposite to the first end, second hole portions which are closed at the first end and are open at the second end, and third hole portions which are closed at the first end. The first hole portion and the second hole portion are adjacent to each other. The third hole portions are adjacent to each other. The PM in the exhaust gas passing through partition walls which partition the first hole portion and the second hole portion adjacent to each other is collected by the partition walls.