This paper proposes a method for rating prediction tasks in review documents. Existing models for predicting the rating scores assigned to each aspect were often developed separately. However, there is a relationship among aspects. Therefore, we apply a multi-task learning framework to our prediction models. We evaluate our model with a review dataset about a game software domain written in Japanese. Each review document contains six-level rating scores for seven aspects of the game software. We utilize BERT as the backbone model for the prediction. Our model simultaneously learns the parameters of seven BERTs for seven aspects. Hence, we name this method “Simultaneous Learning.” It leads to the improvement of the performance of the prediction. In addition, we compare two types of input data for the rating prediction task: all sentences and selected sentences. Experimental results show the effectiveness of our simultaneous learning model in the multi-aspect rating prediction. Our simultaneous learning method obtained 0.713 on RMSE. It demonstrated a 0.046 improvement in RMSE, as compared with non-simultaneous learning. Further, we confirmed that our method based on simultaneous learning was effective in the case that the number of sentences related to an aspect is small.