The present invention relates to an agent which slows down the heart rate which has an excellent controlling ability in diagnostic imaging comprising a short-acting β-blocker (e.g. landiolol hydrochloride or esmolol hydrochloride). The short-acting β-blocker has a property of slowing down the heart rate and it can temporarily suppress the tachycardia at diagnosis. According to the dose and the method of administration, it can control the period for the heart rate adjustment. Also, the present invention relates to a diagnostic imaging auxiliary comprising a short-acting β-blocker as active ingredient.
