An optical multiplexing module, which is easier in adjustment of an optical axis, has an optical multiplexing element and a prism to change optical paths of input light. The module also has one or more input collimators at the input-side end thereof a single output collimator at the output-side end thereof. The optical axes of the collimators are adjustable by optical axis adjusting means. Further, according to the method of correcting the optical axes, after fabricating the collimators into the module, with an optical loss between optical parts monitored, an external force is applied in turn to a plurality of arbitrary locations around a joint portion of one optical part. This application allows a hit location showing a maximum reduction in optical loss to be found out. Radiating a laser beam onto an opposite-side location on the joint portion to the hit location leads to an easier correction of displacements of the optical axis with ease.