An infusion device includes a pump forming front and rear chambers for receiving medicaments therein. The medicaments supplied from the two chambers are respectively transmitted into an elastic reservoir forming first and second lumens through first and second passageways. Then, the medicaments are infused into a person through a catheter which defines one passageway toward the human body. The first passageway is in fluid communication with the front chamber and second lumen. The second passageway is in fluid communication with the rear chamber, the first lumen, and the second lumen. By making the diameter of a second hole which connects the first lumen and second lumen smaller than a first hole which connects the first passageway and the first lumen, the medicament from the second chamber can be infused into patient with a low concentration.