We have investigated the quantum Hall effect breakdown (QHE) by using samples whose confinement potential at the sample boundary is tunable by using side gate structure. The Hall bar shaped samples are made of two types of the GaAs/AlGaAs heterostructure wafers, where the QHE breakdown current is linear or sub-linear to the sample width. In both linear and sub-linear type of samples, the value of breakdown current is scarcely affected by changing the side gate voltage, which suggests that the QHE breakdown is almost independent of the confinement potential at the 2DEG edge. We have measured the QHE breakdown with Corbino shaped samples. The critical voltage in the sub-linear type sample is much smaller (<3 kV/ m) than those in the linear type samples (≈10 kV/ m) , and the width dependence of critical voltage shows nonlinear feature. The origin of the sub-linear type QHE breakdown is not ‘edge effect’ but the properties of bulk two-dimensional electron system.