Relating to real contact area against plane smooth surface, effects of the measuring conditions on paper friction, surrounding atmosphere and applied pressure, were examined. An optical interference method using a scanning laser microscope was adopted to measure the real contact area against a plane smooth surface. The estimated contact area of paper is around 1 of nominal area for common papers. The increase in the contact area with increasing relative humidity is considered as a cause of the increase in the friction coefficient with an increase in relative humidity. The contact area of paper generally increased in a ratio of two-thirds power of the apparent pressure over a wide range of pressure. It suggested that contact of paper was generally elastic and this fact was one reason describing the decrease in the coefficient with increasing apparent pressure. However the decrease in the coefficient is little and the frictional behavior of paper can not be fully described by the contact area against a smooth surface.
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