An oscillator includes a resonator section structured such that a dielectric is interposed between first and second conductors and such that the first and second conductors are electrically connected to a resonant tunneling diode, a capacitor section structured such that the dielectric is interposed between the first and second conductors, a line section configured to electrically connect the resonator section and the capacitor section in parallel to each other, and a resistor section configured to electrically connect the first and second conductors to each other. A first position of the resonator section and a second position of the capacitor section are connected to each other by the line section so that the first position and the second position are substantially electrically equivalent to each other in a wavelength range larger than a wavelength of an electromagnetic wave that resonates in the resonator section.