Abstract: A method of fabricating a sidewall unit includes a step of providing members configuring the sidewall units, a step of forming a first joint structure, a first step of eliminating distortion, a step of forming a second joint structure, and a step of assembling a sidewall unit. The step of eliminating distortion is a step to eliminate distortion due to heat generated when welding members. In the step, a force substantially vertical to a flange surface and a force substantially horizontal to the flange surface are applied to a joint structure before joining the joint structure and a sidewall panel. A fluid storage tank using the sidewall units according to the present invention can securely prevent leakage of fluid even when a large quantity of fluid is stored therein.