We investigated a diode-pumped Cs-based Mx type atomic magnetometer developed aiming the detection of low magnetic fields of the human heartbeat. In this study, we characterized and optimized the rf coil voltage and the laser power to achieve the minimum magnetic resonance linewidth for the best sensitivity. The measured half-width of an 18 Hz at half maximum and a signal-to-noise ratio of 22800 with shot-noise-limited were obtained. The sensitivity of the magnetometer was shown to be around 228 fT/√Hz which is under the required limit for the cardiomagnetometer applications.