Article 23 (Restriction on Dismissal, etc.), Labor Standards Act, as last amended by Act No. 12325, Jan. 21, 2014

An employer has an authority over personnel affairs.
Therefore, labor law does not regulate specific grounds for disciplinary measures.
Instead, it regulates that there must be a “justifiable cause” in the perspective of socially accepted norm.

*Labor Standards Act

Article 23 (Restriction on Dismissal, etc.)(1) An employer shall not, without justifiable cause, dismiss, lay off, suspend, or transfer a worker, reduce his/her wages, or take other punitive measures (hereinafter referred to as "unfair dismissal, etc.") against him/her.

(2) An employer shall not dismiss a worker during a period of suspension of work for medical treatment of an occupational injury or disease and within 30 days immediately thereafter, and any woman before and after childbirth shall not be dismissed during a period of suspension of work as prescribed by this Act and for 30 days immediately thereafter: Provided, That this shall not apply where the employer has paid a lump sum compensation as provided for under Article 84 or where the employer may not continue to conduct his/her business.