This invention relates to a bicyclic nitrogen-containing aromatic heterocyclic amide compound of formula (I), or a salt thereof, which has excellent Complex 1 inhibitory effect and AMPK activating effect, and which is useful as a therapeutic drug for breast cancer. This compound has a cell growth inhibitory effect on a human PIK3CA mutation-positive breast cancer cell line not expressing MCT4 and on a human breast cancer cell line not expressing MCT4 and not having a PIK3CA mutation, and exhibits antitumor action in human PIK3CA mutation-positive breast cancer cell line MDA-MB-453 cell tumor-bearing mice not expressing MCT4.
