A biological information detection device according to an aspect of the present disclosure includes: a light source that, in operation, emits irradiation light for irradiating a test portion of a subject; a light detector that, in operation, detects light reached from the test portion and that outputs an electrical signal corresponding to the light; and a calculation circuit that, in operation, generates a signal of biological information related to a blood flow in a target area in the test portion based on the electrical signal. The light detector is an image sensor. The electrical signal includes an image signal obtained by the image sensor. The calculation circuit, in operation, detects a magnitude of an inclination of an orientation of the test portion with respect to a reference orientation by image recognition based on the image signal, and determines the target area according to the magnitude of the inclination of the orientation of the test portion.