An inbuilt component fixing structure of a resin tank includes: a fastening seat that is provided to the inbuilt component, that is formed with a through-hole configured to fasten and fix the inbuilt component to the resin tank, and that has a contact face that contacts an inner face of the resin tank; a fixing section that projects out from the inner face and pierces through the through-hole, the fastening seat being fastened and fixed by being sandwiched between a jutting-out portion formed at a portion of the fixing section that has pierced through the through-hole, the jutting-out portion jutting out to a peripheral portion of the through-hole; and a weak portion that is formed at the fastening seat, that is thinner than a general portion of the fastening seat, and that has a lower rigidity in a removal direction of the fastening seat than the fixing section.