  In this work, we develop new self-learning techniques with an attention-based
sequence-to-sequence (seq2seq) model for automatic speech recognition (ASR).
For untranscribed speech data, the hypothesis from an ASR system must be used
as a label. However, the imperfect ASR result makes unsupervised learning
difficult to consistently improve recognition performance especially in the
case that multiple powerful teacher models are unavailable. In contrast to
conventional unsupervised learning approaches, we adopt the \emph{multi-task
learning} (MTL) framework where the $n$-th best ASR hypothesis is used as the
label of each task. The seq2seq network is updated through the MTL framework so
as to find the common representation that can cover multiple hypotheses. By
doing so, the effect of the \emph{hard-decision} errors can be alleviated.
  We first demonstrate the effectiveness of our self-learning methods through
ASR experiments in an accent adaptation task between the US and British English
speech. Our experiment results show that our method can reduce the WER on the
British speech data from 14.55\% to 10.36\% compared to the baseline model
trained with the US English data only. Moreover, we investigate the effect of
our proposed methods in a federated learning scenario.
