Deep inspiration breath-hold (DIBH) technique is effective in reducing heart dose compared to a free-breathing technique in the treatment for left side breast cancer. The purpose of the study was to show the workflow and the accuracy of our new irradiation method for it with a combination of a respiration monitoring device and body surface monitoring system. For the workflow of the treatment, first, a patient practiced DIBH during the CT simulation using a respiration monitoring device that could recognize the patient's own respiratory phase using audio or visual guidance. Then, a CT scan was taken under the DIBH condition and treatment plan of tangential two opposed fields was made. In the treatment, patient-setup was performed by adjusting the body surface-scanned image to the reconstructed surface image made of the planning CT images. Audio guidance was used to accomplish the DIBH. For the evaluation of the accuracy of this method, totally 150 irradiations in three patients were analyzed. Inter-fractional positioning errors was calculated in comparison of planning CT images with the fractional CBCT images per a week. Intra-fractional organ motions were calculated in comparison of the digitally reconstructed radiograph (DRR) made of planning CT images with the actual portal MV-cine image taken under DIBH in each fraction. All treatments were completed safely. The average inter-fractional positioning errors calculated form weekly CBCT images were within 2.3 mm for lateral, longitudinal, and vertical directions respectively. The intra-fractional reproducibility of the DIBH was -0.1 ± 2.0 mm and -0.2 ± 1.8 mm (n = 150 beams) in the horizontal and vertical directions, respectively. Our new DIBH method using a new combination of respiration monitoring device and optical body surface monitoring system is effective for left side breast cancer treatment with accurate inter- and intra-fractional reproducibility of the DIBH position.