To obtain a rubber composition excellent in storage stability, a crosslinking rate, and a balance among the mechanical properties of a vulcanizate by mixing an epihalohydrin rubber with a triazinethiol compound and a metaborate of a specified metal.

This composition contains, desirably 100 pts.wt. epihalohydrin rubber, desirably 0.1-5 pts.wt. triazinethiol compound (e.g. 2,4,6-trimercapto-s- triazine), and desirably, 1-20 pts.wt. metaborate of a group II metal in the periodic table (e.g. barium metaborate). It is desirable that the composition additionally contains, desirably, 1-20 pts.wt., per 100 pts.wt. epihalohydrin rubber, hydrotalcite [e.g. Mg4.5Al2(OH)13CO3.3.5 H2O]. The composition is resistant to premature crosslinking, has excellent processing stability, and can give a crosslinked product having a low compression set. It is desirable that the epihalohydrin rubber has a Mooney viscosity (ML1+4, 100°C) of 20-200.