We consider the problem of training an unbiased and accurate model using a biased dataset with multiple biases.One of the major challenges is to balance improving overall accuracy and ignoring all the biases. To address this, we provide a novel framework connecting the problem to multi-task learning (MTL). To be specific, our framework divides training data into several groups according to their effects on the model bias, and defines each task of MTL as solving the target problem for each group. It in turn trains a single model for all the tasks with a weighted sum of task-wise losses as the training objective, while optimizing the weights as well as the model parameters. At the heart of our method lies the weight adjustment algorithm, which is rooted in a theory of multi-objective optimization and guarantees a Pareto-stationary solution. Our algorithm achieved the state of the art on two datasets with multiple biases, and demonstrated superior performance on conventional single-bias datasets.