This paper presents a design of proximity vital sign sensor using a self-oscillating mixer (SOM) at 2.45 GHz. The proposed system consists of a planar resonator that determines the oscillation frequency and a transistor that functions not only as an oscillator but also as a mixer. Note that the supply voltage of the transistor is in pinch-off region to maximize non-linearity. The input impedance of the resonator is varied due to the distance variation between the human body and the resonator. This variation of input impedance changes the oscillation frequency of the proposed sensor. Then, this frequency deviation is self-mixed in the transistor. It can be detected at the emitter in a form of DC voltage, because the proposed system is a kind of direct conversion receiver. So, if we track the emitter voltage, we can obtain the vital sign of the human subject. The measurement results confirm that the proposed sensor can detect the vital sign at 20 mm away from a subject.