The present invention provides an R-T-B based permanent magnet suitable as a magnet with a variable magnetic force for a motor with variable magnetic flux, which has a high residual magnetic flux density, a low coercivity, and a magnetic force that can be reversibly varied by a small external magnetic field. A permanent magnet with a high residual magnetic flux density and a low coercivity that is suitable as a variable magnet for a motor with variable magnetic flux can be obtained, by selecting the rare earth elements consisting of one or more of Y, La and Ce for a predetermined amount of the rare earth element(s) R in the R-T-B based permanent magnet, and further adding a predetermined amount of the additive element(s) which is at least one of Al, Cu, Zr, Hf and Ti.