A diagnostic agent to be used in combination with a contrast agent. To obtain clear images of the coronary artery in diagnostic imaging using multislice helical CT, an agent, whereby about 0.05 mg/kg to about 2 mg/kg of landiolol hydrochloride is intravenously administered in a single administration for 1 minute to a patient showing a high heart rate in a non-operative state, can exhibit a safe and desired effect of improving the coronary artery extractability.