We observe the electron spin resonance of conduction electrons in boron-doped (6400 ppm) superconducting diamond (T-c = 3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent electron spin resonance signal intensity and its magnitude, which is in good agreement with that expected from the density of states through the Pauli spin susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature, which can be understood in the framework of the Elliott-Yafet theory of spin relaxation. An anomalous and yet unexplained relation is observed between the g-factor, CESR linewidth, and the resistivity using the empirical Elliott-Yafet relation.