In this study, we developed an isocenter verification device for use in proton therapy. Radiation and mechanical isocenters were verified for treatment equipment including room lasers, a digital radiography system and the beam axis of a rotational gantry. The special feature of this device is its ability to correlate the position of the three isocenters in one measurement and thus improve accuracy compared to the conventional method using three separate devices. The reproducibility of the method and the fluctuation of the position of the beam axis isocenter were both investigated using this device for almost a year. Monthly measurements of the isocenter position were acquired for two gantries and it was found that the fluctuation was +/- 0.10mm for the up-to-down direction and +/- 0.16mm for the right-to-left direction in Gantry 1 and was +/-0.14mm for the up-to-down direction and +/-0.18mm for the right-to-left direction in Gantry 2. We could be measured with a repeatability of +/-0.18 mm or less by using developed device for the relative positional relationship between each isocenters. Because we can confirm results in approximately 30 minutes, we can perform a quality control after a clinical practice.