There is provided a tire noise reduction device and a pneumatic tire, in which tire vibration can be reduced while durability is maintained and tire noise is further reduced.The tire noise reduction device 1 of the present invention includes two main sound absorbing members 3 for changing the cross-sectional area of a cavity 15 of a tire in the circumferential direction of the tire and two assistant sound absorbing members 4 disposed between the two main sound absorbing members 3. Each of the sound absorbing members 3 and 4 is formed of a porous material. Each main sound absorbing member 3 comprises a first sound absorbing part 3A located on a tread portion inner surface 11a side thereof and a second sound absorbing part 3B located on a side thereof away from the tread portion inner surface 11a. The second sound absorbing part 3B has a density lower than that of the first sound absorbing part 3A. Each assistant sound absorbing member 4 has a density higher than that of the second sound absorbing part 3B and a thickness less than that of the main sound absorbing member 3.