To provide a monitoring apparatus for a fiber bundle that detects the variation in weight of the moving fiber bundle and prevents the leakage of the product with quality abnormality.

The monitoring apparatus for the fiber bundle comprises [A] positioning means moving a fiber bundle in contact with a roller or a fixed guide, [B] output means irradiating the moving fiber bundle with laser light, [C] input means receiving reflected light of the laser light at each point in a width direction of the fiber bundle on a light-receiving body, [D] detecting means detecting a variation amount of the light-receiving angle between the irradiation with the irradiating light and the reflected light at each point in the width direction of the fiber bundle, and [E](i) first data processing means calculating a cross sectional area of the fiber bundle from the thickness of the fiber bundle calculated from the variation amount of the angle at each point in the width direction of the fiber bundle on the basis of the roller or the fixed guide as a standard surface, or (ii) the first data processing means (i) and second data processing means calculating a variation of the cross sectional area obtained from the first data processing means.