An automatic transmission has a transmission control section performing a control so that a transmission ratio defined as input rpm/output rpm of the automatic transmission becomes a target transmission ratio, a slip control section slip-controlling a frictional engagement element in the automatic transmission so that the input rpm becomes a value obtained by multiplying the output rpm by the target transmission ratio also adding a predetermined slip revolution speed; and an abnormality judgment section judging abnormality in the automatic transmission. When the slip-control is not in progress, if an actual transmission ratio is out of a predetermined range of the target transmission ratio, the abnormality judgment section judges that abnormality occurs. When the slip-control is in progress, if a value obtained by correcting the actual transmission ratio on the basis of the slip revolution speed is out of the predetermined range, the abnormality judgment section judges that abnormality occurs.