An objective of the present invention is to provide attenuated minus-strand RNA viruses. The present inventors discovered that the amino acid mutation at position 1214 (Y1214F) in the amino acid sequence of L protein of Sendai virus suppressed the viral genome replication activity and/or transcription activity. The inventors also found that the deletion of a particular gene from the viral genome could result in much less cytotoxicity and immune response than conventional. The inventors thus completed the present invention.