There are provided an oxide dielectric having excellent properties and a solid state electronic device (e.g., a capacitor, a semiconductor device, or a small electromechanical system) having such an oxide dielectric. An oxide layer 30 includes an oxide dielectric (possibly including inevitable impurities) including bismuth (Bi) and niobium (Nb) and having a first crystal phase of a pyrochlore-type crystal structure and a second crystal phase of a ²-BiNbO 4 -type crystal structure. The oxide layer 30 has a controlled content of the first crystal phase and a controlled content of the second crystal phase, in which the first crystal phase has a dielectric constant that decreases with increasing temperature of the oxide layer 30 in a temperature range of 25°C or more and 120°C or less, and the second crystal phase has a dielectric constant that increases with increasing temperature of the oxide layer 30 in the temperature range.