A probe comprising: a light-emitting element having an optical axis and configured to emit light; a light-detecting element having a light-detecting surface configured to detect light emitted from the light-emitting element, and configured to output a signal in accordance with intensity of the detected light; a first support having: a first attachment surface adapted to be attached to a first portion of a living tissue of a subject; a first supporting portion supporting the light-emitting element; and a first passage configured to allow the light emitted from the light-emitting element to pass through; and a second support having: a second attachment surface adapted to be attached to a second portion of the living tissue of the subject; a second supporting portion supporting the light-detecting element; and a second passage configured to allow the light having passed through the second portion to pass through, wherein the light-emitting element is supported by the first supporting portion such that the optical axis is inclined relative to a direction orthogonal to the first attachment surface, and wherein the first attachment surface and the second attachment surface are configured to be attached, facing each other, to the first and second portions of the living tissue, respectively so that: - the light-detecting element is supported by the second supporting portion such that the optical axis extends onto the light detecting surface, so that the light-detecting element detects the light that was emitted by the light-emitting element and transmitted through the living tissue; and - the second passage is located so as not to overlap with the first passage as seen from a direction orthogonal to the second attachment surface. a light-emitting element having an optical axis and configured to emit light; a light-detecting element having a light-detecting surface configured to detect light emitted from the light-emitting element, and configured to output a signal in accordance with intensity of the detected light; a first support having: a first attachment surface adapted to be attached to a first portion of a living tissue of a subject; a first supporting portion supporting the light-emitting element; and a first passage configured to allow the light emitted from the light-emitting element to pass through; a first attachment surface adapted to be attached to a first portion of a living tissue of a subject; a first supporting portion supporting the light-emitting element; and a first passage configured to allow the light emitted from the light-emitting element to pass through; and a second support having: a second attachment surface adapted to be attached to a second portion of the living tissue of the subject; a second supporting portion supporting the light-detecting element; and a second passage configured to allow the light having passed through the second portion to pass through, a second attachment surface adapted to be attached to a second portion of the living tissue of the subject; a second supporting portion supporting the light-detecting element; and a second passage configured to allow the light having passed through the second portion to pass through, wherein the light-emitting element is supported by the first supporting portion such that the optical axis is inclined relative to a direction orthogonal to the first attachment surface, and wherein the first attachment surface and the second attachment surface are configured to be attached, facing each other, to the first and second portions of the living tissue, respectively so that: - the light-detecting element is supported by the second supporting portion such that the optical axis extends onto the light detecting surface, so that the light-detecting element detects the light that was emitted by the light-emitting element and transmitted through the living tissue; and - the second passage is located so as not to overlap with the first passage as seen from a direction orthogonal to the second attachment surface. - the light-detecting element is supported by the second supporting portion such that the optical axis extends onto the light detecting surface, so that the light-detecting element detects the light that was emitted by the light-emitting element and transmitted through the living tissue; and - the second passage is located so as not to overlap with the first passage as seen from a direction orthogonal to the second attachment surface.