Contains a polygonal apse of seven sides surrounded by an ambulatory, without crossing and chapels between the right side buttresses, while in the left there is an open portico.

The pillars change in form and shape depending its function. The capitals are running fascia type putting together individuals threefold arches, built by three decreasing arches.

The whole main nave is surrounded by a triforium. Its forms and important architectural divisions and decorating tracery are proper of the XIV Century. The space between the pillars is cover by an arrangement of trilobular arches over a double quatrefoil (four partially over lapping circles of same diameter. It is a characteristic element of the Basque Gothic.

The naves preset Gothic cross vaulting. The first is a quadripartite rib vault with a central hagiographic motive.

The second with tercels and secondary ribs with a Lys flower decoration.

Getting more complexes, the third presents a complete starred vault with eleven keystones spanding the octagonal shape.

The forth is the more elaborated in starred combinations.

The lateral naves have simple cross vault as the torah arch.