We proposed and fabricated a resonant tunneling diode (RTD) oscillator using a stacked-layer slot antenna toward terahertz range. The proposed antenna is formed with a slot of SiO2 sandwiched between metal layers on a semiconductor substrate. This structure is convenient in obtaining small diffracted radiation and also in extracting most of the output into the air side, compared with conventional slot antennas directly formed on the substrate with high refractive index. Using this antenna integrated with GaInAs/AlInAs RTD on InP substrate, an oscillation at 254 GHz was demonstrated. Theoretical analysis of the oscillation frequency estimated from the admittances of the RTD and antenna is in good agreement with the measurement. Oscillation in the terahertz range is expected by reducing the antenna length and by optimizing the RTD structure.