Assigning labels to instances is crucial for supervised machine learning. In this letter, we propose a novel annotation method, Q&A labeling, which involves a question generator that asks questions about the labels of the instances to be assigned and an annotator that answers the questions and assigns the corresponding labels to the instances. We derived a generative model of labels assigned according to two Q&A labeling procedures that differ in the way questions are asked and answered. We showed that in both procedures, the derived model is partially consistent with that assumed in previous studies. The main distinction of this study from previous ones lies in the fact that the label generative model was not assumed but, rather, derived based on the definition of a specific annotation method, Q&A labeling. We also derived a loss function to evaluate the classification risk of ordinary supervised machine learning using instances assigned Q&A labels and evaluated the upper bound of the classification error. The results indicate statistical consistency in learning with Q&A labels.