A biosensor (100, 200, 300) comprising: a wiring board (160, 260, 360); a light emitting element (121, 122, 221, 222, 321, 322) and a light receiving element (123, 223, 323) 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 (171, 271, 371) having light transmitting properties and configured to seal the light emitting element; a light-receiving-element sealing portion (172, 272, 372) having light transmitting properties and configured to seal the light receiving element; a light shielding portion (173, 273, 373) disposed between the light-emitting-element sealing portion and the light-receiving-element sealing portion; a base medium (150, 250, 350) having light transmitting properties, disposed in parallel with the wiring board with the light shielding portion therebetween; and an adhesion layer (180, 185, 280, 380) 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 the light-emitting-element sealing portion (171, 271, 371) and the light-receiving-element sealing portion (172, 272, 372) each have a first surface disposed on the principal surface of the wiring board (160, 260, 360), and a second surface disposed at the adhesion layer (180, 185, 280, 380). a wiring board (160, 260, 360); a light emitting element (121, 122, 221, 222, 321, 322) and a light receiving element (123, 223, 323) 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 (171, 271, 371) having light transmitting properties and configured to seal the light emitting element; a light-receiving-element sealing portion (172, 272, 372) having light transmitting properties and configured to seal the light receiving element; a light shielding portion (173, 273, 373) disposed between the light-emitting-element sealing portion and the light-receiving-element sealing portion; a base medium (150, 250, 350) having light transmitting properties, disposed in parallel with the wiring board with the light shielding portion therebetween; and an adhesion layer (180, 185, 280, 380) 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 the light-emitting-element sealing portion (171, 271, 371) and the light-receiving-element sealing portion (172, 272, 372) each have a first surface disposed on the principal surface of the wiring board (160, 260, 360), and a second surface disposed at the adhesion layer (180, 185, 280, 380).