A spin-wave theory is developed for the canting antiferromagnetism by using the Holstein-Primakoff transformation. The correction arising from interactions between spin waves can be obtained easily in this theory. The theory is applied to study the field-induced transition in La₂CuO₄ when the magnetic field is perpendicular to the CuO₂ plane. The temperature dependence of the critical magnetic field predicted by our theory agrees fairly well with experiment at low temperatures. The correction terms are quite small at low temperatures, where the theory fits experiment well. This means that the simple theory, neglecting spin-wave interactions, is sufficient for practical purposes.