We present a highly sensitive miniaturized cavity-enhanced room-temperature magnetic-field sensor based on nitrogen-vacancy (NV) centers in diamond. The magnetic resonance signal is detected by probing absorption on the 1042,nm spin-singlet transition. To improve the absorptive signal the diamond is placed in an optical resonator. The device has a magnetic-field sensitivity of 28 pT/$sqrt{rm{Hz}}$, a projected photon shot-noise-limited sensitivity of 22 pT/$sqrt{rm{Hz}}$ and an estimated quantum projection-noise-limited sensitivity of 0.43 pT/$sqrt{rm{Hz}}$ with the sensing volume of $sim$ 390 $mu$m $times$ 4500 $mu$m$^{2}$. The presented miniaturized device is the basis for an endoscopic magnetic field sensor for biomedical applications.
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