A Monolithic Eulerian Fluid-Structure Interaction Scheme [1] uses spatially averaged equations of the governing equations of motion of the fluid and structure to solve fluid-structure interaction problems. This method has advantages in terms of its high computational performance in a massively parallel environment and handling of large deformations of solids. In this method, motions of fluids and solids are analyzed on a mesh that is fixed in space and does not deform. This makes it difficult to assign velocity boundary conditions to arbitrary positions and shapes in a computational domain. Furthermore, in numerical analysis, the available time increments are limited by the CFL condition for solid stress waves, so the time increments must be determined by materials with high Young's modulus. Thus, when deformation of a material with a high Young's modulus is very small and deformation of a material with a low Young's modulus is only interested in, treating the high Young's modulus material as a rigid body makes it possible to ignore the deformation of the material with a high Young's modulus and avoid the time step restriction. In this presentation, we propose a method to solve these problems by introducing spatially averaged equations of motions with a constraint based immersed boundary method [2], and show computational results using the proposed method. REFERENCES [1] T. Shimada, K. Nishiguchi, R. Bale, S. Okazawa and M. Tsubokura, “Eulerian finite volume formulation using Lagrangian marker particles for incompressible fluid–structure interaction problems”, International Journal for Numerical Methods in Engineering, Vol. 123, No. 5, 1294-1328 (2022) [2] A.A. Shirgaonkar, M.A. MacIver and N.A. Patankar, “A new mathematical formulation and fast algorithm for fully resolved simulation of self-propulsion”, Journal of Computational Physics Vol. 228, No. 7, 2366–2390 (2009)