To adjust the loss of an optical fiber with simple constitution.

This loss adjustor is composed of a fiber fixing substrate 1, a press cap 2 and a fiber or array fiber 3. Coating of a middle part is removed, and glass of a fiber is exposed to be placed on the fiber fixing substrate 1. The fiber fixing substrate 1 is performed with groove 4 processing on the central part of the fiber fixing substrate so as to be orthogonally intersected with the longitudinal axis of the fiber when the fiber is placed. Heating is used for adjusting the loss. By heating the fiber, the fiber is melted, and a bend occurs in a linear fiber, and an optical loss occurs. When the fiber is adjusted once, the loss is stabilized permanently. Since an attenuation membrane and a filter, etc., aren't inserted into the optical path of the fiber, reliability is improved.