In radiation therapy for glottic cancer, respiratory motion of larynx may change the dose variation in the target. The purpose of this study is to measure the respiratory motion of the larynx, and quantify the impact of the motion on the dose variation. This study included 10 patients treated by opposing portal irradiation for glottic cancer. We acquired fluoroscopy and respiratory waveform of the patients simultaneously and formulated the relationship between the displacement of larynx and the respiratory phase. We divided one field into 39 sub-fields on the basis of control points. Dose distributions accounting for the displacement were calculated by shifting isocenter calculated using the formula in every sub-fields. Dose variations of clinical target volume (CTV) were evaluated by subtracting dose distributions with displacement consideration and dose distributions without it. Average amplitude and the maximum amplitude of respiratory motion were 2.5 and 8.7 mm, respectively. Average of mean dose variation in CTV was 0.1% of the prescribed dose, and maximum of local dose variation was 2.0% of the prescribed dose. Hence, it is realized that dose variation in CTV by respiratory motion was slight.