An EGR system for an internal combustion engine, characterized by comprising: a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; a high-pressure EGR valve that is provided in the high-pressure EGR passage, and that changes a flow passage area of the high-pressure EGR passage; a low-pressure EGR valve that is provided in the low-pressure EGR passage, and that changes a flow passage area of the low-pressure EGR passage; and an EGR control unit that adjusts an opening amount of the high-pressure EGR valve to a reference high-pressure EGR valve opening amount determined based on an operation mode of the internal combustion engine, and adjusts an opening amount of the low-pressure EGR valve to a reference low-pressure EGR valve opening amount determined based on the operation mode of the internal combustion engine, wherein, in a transitional period in which the operation mode of the internal combustion engine is changing from a predetermined first operation mode to a second operation mode in which a load, which is higher than a load in the first operation mode, is applied to the internal combustion engine, the EGR control unit adjusts the opening amount of the high-pressure EGR valve to an opening amount that is smaller than a reference high-pressure EGR valve opening amount corresponding to the second operation mode, the EGR system further comprising: an engine speed-up prediction unit that predicts whether the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode, wherein, when the engine speed-up prediction unit predicts that the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode, the EGR control unit adjusts the opening amount of the high-pressure EGR valve to an opening amount that is larger than a reference high-pressure EGR valve opening amount corresponding to the first operation mode so that a ratio of an amount of exhaust gas, recirculated back to the internal combustion engine by the high-pressure EGR unit, to a total amount of exhaust gas, recirculated back to the internal combustion engine by the high-pressure EGR unit and the low-pressure EGR unit, is higher than that before the engine speed-up prediction unit predicts that the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode. a turbocharger that includes a compressor arranged in an intake passage of the internal combustion engine, and a turbine arranged in an exhaust passage of the internal combustion engine; a high-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a high-pressure EGR passage that provides communication between the exhaust passage, at a portion upstream of the turbine, and the intake passage, at a portion downstream of the compressor; a low-pressure EGR unit that recirculates a portion of exhaust gas back to the internal combustion engine through a low-pressure EGR passage that provides communication between the exhaust passage, at a portion downstream of the turbine, and the intake passage, at a portion upstream of the compressor; a high-pressure EGR valve that is provided in the high-pressure EGR passage, and that changes a flow passage area of the high-pressure EGR passage; a low-pressure EGR valve that is provided in the low-pressure EGR passage, and that changes a flow passage area of the low-pressure EGR passage; and an EGR control unit that adjusts an opening amount of the high-pressure EGR valve to a reference high-pressure EGR valve opening amount determined based on an operation mode of the internal combustion engine, and adjusts an opening amount of the low-pressure EGR valve to a reference low-pressure EGR valve opening amount determined based on the operation mode of the internal combustion engine, an engine speed-up prediction unit that predicts whether the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode, wherein, when the engine speed-up prediction unit predicts that the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode, the EGR control unit adjusts the opening amount of the high-pressure EGR valve to an opening amount that is larger than a reference high-pressure EGR valve opening amount corresponding to the first operation mode so that a ratio of an amount of exhaust gas, recirculated back to the internal combustion engine by the high-pressure EGR unit, to a total amount of exhaust gas, recirculated back to the internal combustion engine by the high-pressure EGR unit and the low-pressure EGR unit, is higher than that before the engine speed-up prediction unit predicts that the operation mode of the internal combustion engine is changed from the first operation mode to the second operation mode.