An antenna device (60) is provided that is used in a recorder and/or writer destined for writing and reading data to and from a contactless IC card (1). The antenna device (60) includes a loop coil (61) that radiates a magnetic field, magnetically couples with a loop coil (4) provided in the IC card (1), and sends and receives data to and from the IC card (1). The loop coil (61) is formed asymmetric for the winding sections thereof opposite to each other across the center of the loop coil (61) to be different in interval from each other.