A fuel tank has a built-in component with a head portion, a neck portion, and a shoulder portion, and has the built-in component anchored to a tank body with a parison wrapped around the neck portion during molding. The fuel tank includes a deformation prevention structure configured to prevent a wrapping parison portion wrapped around the neck portion from being deformed due to pressure acting on the tank body. The deformation prevention structure is configured to include one of a stepped portion provided in the shoulder portion, a convex portion, and a recess.