A novel resonant tunneling diode (RTD) oscillator is proposed, which overcomes the problems of the conventional RTD oscillators, such as the low-frequency spurious oscillation and the bias instability. Our proposal consists of two RTDs connected serially, and the resonator connected to the node between two RTDs. This circuit separates the oscillation node from the bias nodes, and suppresses the above mentioned problems. This relaxes the severe restriction on the RTD area, and makes it possible to supply higher power to a load. Circuit simulation shows that with this circuit more than 2 mW power can be supplied to the 50 Ω resistive load at 100 GHz using RTDs having 105 A/cm2-peak current density and 20 µm2-area. It also shows that the dc-to-RF conversion efficiency is as good as that of conventional ones. Furthermore, we have studied the extension of this oscillator having 4 RTDs connected serially. Circuit simulations revealed that using this circuit the power can be doubled with a good conversion efficiency.