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We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap would be suitable for in-beam PET, which is a method for in situ monitoring of charged particle therapy. In the present work, we developed the first prototype of the OpenPET, especially for in-beam experiments using phantoms. The OpenPET prototype was designed with 2 detector rings of 8 detector blocks. Ring diameter was 110 mm and the axial FOV was 126 mm. Distance between detector rings is 42 mm and could be controlled. The DOI detector consisted of 784 LGSO crystals which were arranged in a 4-layer design of 14 × 14 arrays, coupled to a 64-ch PS-PMT. Each crystal element was 2.9 × 2.9 × 5 mm3. After 64-channel gain correction of the PS-PMT, output signals of two DOI detectors stacked axially were projected on one 2-dimensional position histogram by an Anger calculation. The system sensitivities were measured from a 22Na point source. The system sensitivity was 5.5 %. A count rate performance test was done that utilized a cylinder phantom (4 cm diameter, 8 cm length). Maximum count rate of prompt was 28 kcps at 3.5 MBq. The OpenPET prototype has the promise of high sensitivity and sufficient performance for testing in in-beam experiments.