  The effect of the four-spin cyclic exchange interaction at each plaquette in
the $S=1/2$ two-leg spin ladder is investigated at T=0, especially focusing on
the field-induced gap. The strong rung coupling approximation suggests that it
yields a plateau at half of the saturation moment ($m=1/2$) in the
magnetization curve, which corresponds to a field-induced spin gap with a
spontaneous breaking of the translational symmetry. A precise phase diagram at
$m=1/2$ is also presented based on the level spectroscopy analysis of the
numerical data obtained by Lanczos method. The boundary between the gapless and
plateau phases is confirmed to be of the Kosterlitz-Thouless (KT) universality
class.
