We describe our newly-developed open-source software, named by Jx, to perform magnetic force linear response calculations. Jx is a user-friendly and efficient tool to calculate magnetic interaction in solids and molecules. Without supercell calculation, it computes both short- and long-range interactions. It is also possible to calculate an orbital-resolved matrix form of magnetic couplings. Functionality and formalism are presented with examples. The program architecture and parallel algorithm are also described. Program summary: Program Title: Jx Program Files doi: http://dx.doi.org/10.17632/kg5x95gwgj.1 Licensing provisions: LGPL Programming language: Julia Nature of problem: The first-principles estimation of magnetic interaction in solids and molecules. Solution method: Calculate the magnetic coupling constant based on magnetic force response theory.