We have investigated the polarity of the terahertz electromagnetic wave from GaAs1-xNx epitaxial layers with x=0.43% and 1.53% in order to clarify the effects of nitrogen incorporation on the direction of the surface band bending using an undoped GaAs/n-type GaAs(i-GaAs/n-GaAs) epitaxial layer structure and a semi-insulating GaAs (SI-GaAs) crystal as reference samples. A numerical calculation on the basis of the Boltzmann–Poisson model clarifies that the conduction band of the i-GaAs/n-GaAs sample bends upward at the surface region, which indicates that photogenerated electrons flow into the inside. In the measurement of the terahertz wave, it is observed that the polarity of the terahertz wave from the SI-GaAs crystal is the same as that of the i-GaAs/n-GaAs sample, namely, the present SI-GaAs crystal has an upward band bending. In contrast, the terahertz-wave polarity is reversed in the GaAs1-xNx samples in spite of the relatively small nitrogen mole fraction; namely, the GaAs1-xNx samples have a downward band bending. The above-mentioned terahertz-wave polarity reversal is attributed to the phenomenon that conduction-band bottom is considerably lowered by the band anticrossing peculiar to GaAs1-xNx, which results in approaching the conduction-band bottom to the surface Fermi level. This modifies the direction of the surface band bending connected with the polarity of the terahertz wave.