A gasket having a core member having an opening, an inner seal portion made of elastic material and formed along an inner circumference of the opening, an outer seal portion made of elastic material and formed along an outer circumference of the core member. The core member has a seal-forming thin portion and an easily-deformable portion, the seal-forming thin portion being configured to form the inner seal portion at the inner circumference and to form the outer seal portion at the outer circumference, and the easily-deformable portion being provided between the inner seal portion and the outer seal portion and being configured to easily deform the core member.