A shock absorber has a piston formed with a rebound channel and a valve mechanism for generating a damping force by controlling opening and closing and an opening degree of the rebound channel. The valve mechanism comprises a first valve disk having a hole and a slit communicating with the rebound channel respectively provided in inner and outer sides of the first valve disk, the first valve disk generating a first damping force by directing fluid through the slit; an auxiliary disk having a hole communicating with the hole of the first valve disk, the auxiliary disk being disposed to be in contact with a lower side of the first valve disk; and a second valve disk disposed to be in contact with a lower side of the auxiliary disk, the second valve disk being subjected to bending deformation by the fluid flowing through the holes of the first valve disk and the auxiliary disk to generate a second damping force.