A waveguide sensor according to an embodiment of the present invention includes: a substrate; a first underclad arranged on one side of the substrate; a first sensing core arranged on outer side of the first underclad and having a stripe pattern which extends in one direction; a first overclad arranged on outer side of the first sensing core; a second underclad arranged on another side of the substrate; a second sensing core arranged on outer side of the second underclad and having a stripe pattern which extends in a direction not parallel to the direction in which the first sensing core extends; and a second overclad arranged on outer side of the second sensing core. A first grooved part which extends in a direction not parallel to the direction in which the first sensing core extends is formed on the first overclad, so that the first grooved part and the first sensing core together form a first grating in a plane view. Furthermore, a second grooved part which extends in a direction not parallel to the direction in which the second sensing core extends is formed on the second overclad, so that the second grooved part and the second sensing core together form a second grating in a plane view.