A distinctive magnetic structure in which azimuthal and latitudinal angles of field vectors are closely related to each other has been found in the interplanetary magnetic field data obtained by Sakigake at 0.8--1.0 AU. In this structure, termed a ''planar magnetic structure'' (PMS), the magnetic field vectors are nearly parallel to a fixed plane. This plane includes the spiral direction but is inclined to the ecliptic plane from 30¿ to 85¿. The field vectors take almost all directions parallel to this plane. The PMS consists of several segments in which field directions are almost constant, and the segments are separated by tangential discontinuities where directional changes of the field vector occur abruptly without showing any preferred polarization. The ion number density, the ion temperature, and the plasma &bgr; tend to be higher in the PMS than in the surrounding plasma. The PMS events are clearly distinct from ''magnetic clouds'' both in the field configuration and in the plasma conditions. During the 25 month period from July 1985 to July 1987, eight PMS events with durations of several hours have been identified. The PMS events may be associated with newly emerging magnetic structure in the photosphere from which magnetic tongues are extended into interplanetary space. ¿ American Geophysical Union 1989