This paper presents the development of a variable elastic band that can continuously adjust its elasticity and a hip flexion assistance suit utilizing the band. By controlling the length of the nylon webbing in the band, the elasticity of the band can be changed continuously. The test results show that the band can produce different elastic force profiles according to the length of the webbing. The hip flexion assistance suit using the variable elastic band also demonstrated the ability to apply various levels of force to the wearer. The proposed system enables a human-in-the-loop approach for optimizing the elasticity for individual users and activities, which was difficult to achieve in previous passive systems.