In this paper, we propose a shape computation algorithm for quasi-static shape transition of a planar closed elastica which has a function as an impulse force generator utilizing a snap-through buckling of an elastic strip. The proposed algorithm has an advantage to compute the shape of the closed elastic more stably and quickly than the one to which the shooting method simply applied since it is based on mechanistic equations where an unstable computation factor has been eliminated. The proposed algorithm is written in the forms extensible to shape computation for spatial closed elastica. This good property is based on our modeling where a continuum elastic strip is approximated by a serial chain of multiple rigid links with elastic joints familiar with robotics. Effectiveness of the proposed algorithm is verified by shape transition simulation for planar closed elastica including snap-through buckling usually difficult to compute due to its instable nature. An example of shape transition simulation for spatial closed elastica is also shown for illustrating extension to 3-dimensional cases.