We proposed a noise reduction method for in-vehicle heartbeat sensor systems. The system measures the driver's heartbeat using a steering wheel electrode and a seat electrode that has dual construction. This configuration allows measurement while driving with one hand and provides two signals for noise cancellation. However, the amplitude ratio of the common mode noise varies when driving at high speeds. We multiplied a coefficient that was derived from the Root Mean Square (RMS) ratio between the two signals. This method enabled us to obtain 80% or more of the heartbeat signal while driving at high speed with one hand.