A biological component estimation apparatus, comprising: a light detector array, in particular, a linear light detector array, including a plurality of light detectors; a plurality of light sources comprising a first light source disposed at a first end of the light detector array and a second light source disposed at a second end of the light detector array; and a processor configured to calculate a blood vessel alignment index, which indicates a degree of alignment of the biological component estimation apparatus with respect to blood vessels of a user, based on an intensity of a first light that is emitted from the first light source and measured by the light detector array when the first light is returned from the user, and an intensity of a second light that is emitted from the second light source to the user and measured by the light detector array when the second light is returned from the user, wherein the processor is further configured to calculate: based on the measured intensity of the first light, an intensity slope of the first light which indicates a change in the measured intensity of the first light according to a distance between the first light source and each of the plurality of light detectors; based on the measured intensity of the second light, an intensity slope of the second light which indicates a change in the measured intensity of the second light according to a distance between the second light source and each of the plurality of light detectors; and the blood vessel alignment index as an absolute value of a difference between the intensity slope of the first light and the intensity slope of the second light. a light detector array, in particular, a linear light detector array, including a plurality of light detectors; a plurality of light sources comprising a first light source disposed at a first end of the light detector array and a second light source disposed at a second end of the light detector array; and a processor configured to calculate a blood vessel alignment index, which indicates a degree of alignment of the biological component estimation apparatus with respect to blood vessels of a user, based on an intensity of a first light that is emitted from the first light source and measured by the light detector array when the first light is returned from the user, and an intensity of a second light that is emitted from the second light source to the user and measured by the light detector array when the second light is returned from the user, wherein the processor is further configured to calculate: based on the measured intensity of the first light, an intensity slope of the first light which indicates a change in the measured intensity of the first light according to a distance between the first light source and each of the plurality of light detectors; based on the measured intensity of the second light, an intensity slope of the second light which indicates a change in the measured intensity of the second light according to a distance between the second light source and each of the plurality of light detectors; and the blood vessel alignment index as an absolute value of a difference between the intensity slope of the first light and the intensity slope of the second light. based on the measured intensity of the first light, an intensity slope of the first light which indicates a change in the measured intensity of the first light according to a distance between the first light source and each of the plurality of light detectors; based on the measured intensity of the second light, an intensity slope of the second light which indicates a change in the measured intensity of the second light according to a distance between the second light source and each of the plurality of light detectors; and the blood vessel alignment index as an absolute value of a difference between the intensity slope of the first light and the intensity slope of the second light.