A boot assembly comprising a constant velocity universal joint and a boot, the constant velocity universal joint including: an outer joint member, an inner joint member, and a torque transmitting member interposed between the outer joint member and the inner joint member, for transmitting torque, the boot comprising: a large diameter portion fitted to the outer joint member: a small diameter portion configured to be fitted to a shaft coupled to the inner joint member; a bellows portion connected to a side of the large diameter portion of the small diameter portion, alternately including a peak portion and a root portion (12, 13) which respectively have a peak shape and a root shape radially outward, and forming a root portion on a side which is closest to the large diameter portion; and a shoulder portion for connecting the large diameter portion and the root portion on the side which is closest to the large diameter portion to each other, wherein: in a state where the boot is fitted to the constant velocity universal joint and the shaft at an operating angle of 0 degrees, a circular arc portion and a linear portion are formed over the shoulder portion and the root portion on the side which is closest to the large diameter portion, the circular arc portion being adjacent to the large diameter portion and having an axial sectional shape protruding radially outward, the linear portion being adjacent to the circular arc portion and having a linear axial sectional shape, an inner peripheral surface of the linear portion is in contact with an opening end portion of the outer joint member, and a thickness from the circular arc portion to the linear portion is uniform, the axial sectional shape of the circular arc portion is formed into a semicircular arc shape protruding radially outward, a thermoplastic polyester elastomer is used as a material of the boot, the constant velocity universal joint is a plunging constant velocity universal joint; and even when the boot is subjected to the axial force so as to be compressed and deformed, the deformation of the periphery of the root portion on the side which is closest to the large diameter portion is suppressed, as a result, the root portion on the side which is closest to the large diameter portion is prevented from being moved inside the outer joint member. an outer joint member, an inner joint member, and a torque transmitting member interposed between the outer joint member and the inner joint member, for transmitting torque, the boot comprising: a large diameter portion fitted to the outer joint member: a small diameter portion configured to be fitted to a shaft coupled to the inner joint member; a bellows portion connected to a side of the large diameter portion of the small diameter portion, alternately including a peak portion and a root portion (12, 13) which respectively have a peak shape and a root shape radially outward, and forming a root portion on a side which is closest to the large diameter portion; and a shoulder portion for connecting the large diameter portion and the root portion on the side which is closest to the large diameter portion to each other, a large diameter portion fitted to the outer joint member: a small diameter portion configured to be fitted to a shaft coupled to the inner joint member; a bellows portion connected to a side of the large diameter portion of the small diameter portion, alternately including a peak portion and a root portion (12, 13) which respectively have a peak shape and a root shape radially outward, and forming a root portion on a side which is closest to the large diameter portion; and a shoulder portion for connecting the large diameter portion and the root portion on the side which is closest to the large diameter portion to each other, wherein: in a state where the boot is fitted to the constant velocity universal joint and the shaft at an operating angle of 0 degrees, a circular arc portion and a linear portion are formed over the shoulder portion and the root portion on the side which is closest to the large diameter portion, the circular arc portion being adjacent to the large diameter portion and having an axial sectional shape protruding radially outward, the linear portion being adjacent to the circular arc portion and having a linear axial sectional shape, an inner peripheral surface of the linear portion is in contact with an opening end portion of the outer joint member, and a thickness from the circular arc portion to the linear portion is uniform, the axial sectional shape of the circular arc portion is formed into a semicircular arc shape protruding radially outward, a thermoplastic polyester elastomer is used as a material of the boot, the constant velocity universal joint is a plunging constant velocity universal joint; and even when the boot is subjected to the axial force so as to be compressed and deformed, the deformation of the periphery of the root portion on the side which is closest to the large diameter portion is suppressed, as a result, the root portion on the side which is closest to the large diameter portion is prevented from being moved inside the outer joint member.