A biosensor comprising: a wiring board; a light emitting element (121, 122) and a light receiving element disposed on a principal surface of the wiring board with a predetermined spacing between the light emitting element and the light receiving element; a light-emitting-element sealing portion having light transmitting properties, disposed on the principal surface of the wiring board and configured to seal the light emitting element; a light-receiving-element sealing portion having light transmitting properties, disposed on the principal surface of the wiring board and configured to seal the light receiving element; a light shielding portion disposed between the light-emitting-element sealing portion and the light-receiving-element sealing portion; a base medium having light transmitting properties, disposed in parallel with the wiring board with the light shielding portion therebetween; and an adhesion layer having light transmitting properties, disposed between the base medium and the light shielding portion and/or the light-emitting-element sealing portion and the light-receiving-element sealing portion, and configured to bond the base medium with the light shielding portion and/or the light-emitting-element sealing portion and the light-receiving-element sealing portion, wherein: an end portion of the adhesion layer and an end portion of the base medium closer to the light receiving element are disposed such that the end portions do not overlap the light-receiving-element sealing portion when viewed from a direction normal to the principal surface of the wiring board, or an end portion of the adhesion layer and an end portion of the base medium closer to the light emitting element (121, 122) are disposed such that the end portions do not overlap the light-emitting-element sealing portion when viewed from a direction normal to the principal surface of the wiring board. a wiring board; a light emitting element (121, 122) and a light receiving element disposed on a principal surface of the wiring board with a predetermined spacing between the light emitting element and the light receiving element; a light-emitting-element sealing portion having light transmitting properties, disposed on the principal surface of the wiring board and configured to seal the light emitting element; a light-receiving-element sealing portion having light transmitting properties, disposed on the principal surface of the wiring board and configured to seal the light receiving element; a light shielding portion disposed between the light-emitting-element sealing portion and the light-receiving-element sealing portion; a base medium having light transmitting properties, disposed in parallel with the wiring board with the light shielding portion therebetween; and an adhesion layer having light transmitting properties, disposed between the base medium and the light shielding portion and/or the light-emitting-element sealing portion and the light-receiving-element sealing portion, and configured to bond the base medium with the light shielding portion and/or the light-emitting-element sealing portion and the light-receiving-element sealing portion, wherein: an end portion of the adhesion layer and an end portion of the base medium closer to the light receiving element are disposed such that the end portions do not overlap the light-receiving-element sealing portion when viewed from a direction normal to the principal surface of the wiring board, or an end portion of the adhesion layer and an end portion of the base medium closer to the light emitting element (121, 122) are disposed such that the end portions do not overlap the light-emitting-element sealing portion when viewed from a direction normal to the principal surface of the wiring board. an end portion of the adhesion layer and an end portion of the base medium closer to the light receiving element are disposed such that the end portions do not overlap the light-receiving-element sealing portion when viewed from a direction normal to the principal surface of the wiring board, or an end portion of the adhesion layer and an end portion of the base medium closer to the light emitting element (121, 122) are disposed such that the end portions do not overlap the light-emitting-element sealing portion when viewed from a direction normal to the principal surface of the wiring board.