We are developing an X-ray SOI pixel detector “XRPIX” for the next generation X-ray astronomical satellite “FORCE”. XRPIX is the detector using SOI (Silicon-On-Insulator) technology which makes it possible to integrate a high-resistivity Si sensor part and a low-resistivity Si CMOS circuit part. The CMOS circuit is equipped with the trigger function, which can read out only the output signal of the pixel where the X-ray is incident. This function realizes high throughput and high time resolution, enabling the background rejection with anticoincidence technique. A new series of XRPIX named XRPIX6E, we developed, with a Pinned Depleted Diode (PDD) structure improves the spectral performance by suppressing the interference between the sensor layer and the circuit layer. When semiconductor X-ray detectors are used in space, it is known that their spectral performance is degraded due to radiation damage caused by high-energy protons. Therefore, before using XRPIX in space, it is necessary to evaluate how much the spectral performance will be degraded by radiation damage. Then we performed proton irradiation experiment for XRPIX6E for the first time at HIMAC in National Institute of Radiological Sciences. We irradiated XRPIX with high-energy protons up to a total dose of 40 krad, equivalent to 400 years irradiation in orbit. As a result, the energy resolution in full width half maximum at the 5:9 keV degrades by 25 ± 3%, however, is better than the required performance of FORCE, 300 eV at 6 keV. It was also found that the PDD structure XRPIX has better radiation hardness than the previous XRPIX series. In addition, We investigated about the degradation of the energy resolution; it was found that the degradation would be due to increasing energy independent components, for example, readout noise.