PURPOSE: To industrially produce a high-performance roving exhibiting a high strength, excellent in impregnation with a resin, free from a trouble such as hang-up in drawing and capable of chopping with an excellent spreading and of providing an excellent-quality cloth.

CONSTITUTION: The end parts of two glass fiber bundles 1 and 2 are put in layers and inserted into a passage 4 having a circular cross section. Compressed air is blown through an air inlet 5 perpendicular to the passage so as to open the above-mentioned end parts of the glass fiber bundles and to mutually mix the glass fiber bundles. Thereby, both the glass fiber bundles are bound and a combined glass fiber bundle is formed. Plural combined glass fiber bundles are arranged in a state where the combined parts are mutually shifted in the longitudinal direction.