The objective of the present invention is to efficiently and inexpensively mass-produce a sound insulator for a vehicle which is light in weight and has excellent sound insulating properties. For this purpose, the ultra-light sound insulator of the present invention is composed of a felt single sheet 3 having a vehicle interior side surface 1 and a vehicle exterior side surface 2 and being thermoformed of cotton fibers and binder fibers which are tangled and contacted and jointed to each other in a random manner. The ratio of the stiffness of the vehicle interior side surface 1 to that of the vehicle exterior side surface 2 is set to be in a range of 1.1 to 10. The single sheet 3 also has an area of gradually-decreasing stiffness 4 that spreads over at least one part of the area between the vehicle interior side surface 1 and the vehicle exterior side surface 2. The area of gradually-decreasing stiffness 4 has a stiffness distribution pattern in which the stiffness decreases gradually from the vehicle interior side surface 1 toward the vehicle exterior side surface 2 with respect to the direction perpendicular to the vehicle exterior side surface 2.