A weakly crosslinked sealing material for use in a solar cell module, comprising: all layers of a monolayer or multilayer film comprising a resin composition containing at least 90 mass% of a polyethylene-based resin with a density in a range of 0.870 g/cm 3 to 0.900 g/cm 3 inclusive and containing substantially no crosslinking coagent, the monolayer or multilayer film having a MFR from 0.1 g/10 min to less than 1.0 g/10 min as measured, with a method as follows: the weakly crosslinked sealing material having a gel fraction within 0% and 1% as measured with a method as follows: a gel fraction (%): after crosslinked, 1 g of the sealing material is weighed and placed in an 80-mesh metal gauze bag; subsequently, the sample in the metal gauze is added to a Soxhlet extractor and refluxed with xylene at the boiling point; after continuous extraction for 10 hours, the sample in the metal gauze bag is taken out, and the sample is dried and then weighed; the weights of the sample before and after the extraction are compared to determine the mass percentage (%) of the insoluble residue, which is used as the gel fraction, MFR (g/10 min) is measured according to JIS K 7210, wherein synthetic resin is heated and pressurized at 190°C in a cylindrical vessel heated by a heater, and the amount of the resin extruded per 10 minutes from an opening (nozzle) formed at the bottom of the vessel is measured; the testing machine used is an extrusion plastometer, and the extrusion load is 2.16 kg; concerning a sealing material sheet made of a multilayer film, the above measurement process is performed on a multilayer state in which all layers are laminated, and the resulting measured value is used as the MFR value of the multilayer sealing material sheet. the weakly crosslinked sealing material having a gel fraction within 0% and 1% as measured with a method as follows: a gel fraction (%): after crosslinked, 1 g of the sealing material is weighed and placed in an 80-mesh metal gauze bag; subsequently, the sample in the metal gauze is added to a Soxhlet extractor and refluxed with xylene at the boiling point; after continuous extraction for 10 hours, the sample in the metal gauze bag is taken out, and the sample is dried and then weighed; the weights of the sample before and after the extraction are compared to determine the mass percentage (%) of the insoluble residue, which is used as the gel fraction, MFR (g/10 min) is measured according to JIS K 7210, wherein synthetic resin is heated and pressurized at 190°C in a cylindrical vessel heated by a heater, and the amount of the resin extruded per 10 minutes from an opening (nozzle) formed at the bottom of the vessel is measured; the testing machine used is an extrusion plastometer, and the extrusion load is 2.16 kg; concerning a sealing material sheet made of a multilayer film, the above measurement process is performed on a multilayer state in which all layers are laminated, and the resulting measured value is used as the MFR value of the multilayer sealing material sheet. a gel fraction (%): after crosslinked, 1 g of the sealing material is weighed and placed in an 80-mesh metal gauze bag; subsequently, the sample in the metal gauze is added to a Soxhlet extractor and refluxed with xylene at the boiling point; after continuous extraction for 10 hours, the sample in the metal gauze bag is taken out, and the sample is dried and then weighed; the weights of the sample before and after the extraction are compared to determine the mass percentage (%) of the insoluble residue, which is used as the gel fraction, MFR (g/10 min) is measured according to JIS K 7210, wherein synthetic resin is heated and pressurized at 190°C in a cylindrical vessel heated by a heater, and the amount of the resin extruded per 10 minutes from an opening (nozzle) formed at the bottom of the vessel is measured; the testing machine used is an extrusion plastometer, and the extrusion load is 2.16 kg; concerning a sealing material sheet made of a multilayer film, the above measurement process is performed on a multilayer state in which all layers are laminated, and the resulting measured value is used as the MFR value of the multilayer sealing material sheet.