Extract minute bleeding in the small intestine with high sensitivity and easily detect the presence of bleeding in patients with unknown bleeding. An illumination unit (2) that generates two types of illumination light having different wavelength bands, and an imaging unit (3) each having sensitivity in the wavelength bands of the two types of illumination light from the illumination unit (2), For the illumination light of the first wavelength band from the part (2), the blood and the background tissue have a reflectance higher than a predetermined threshold value, and for the illumination light of the second wavelength band, the blood is less than the threshold value. An in-vivo observation device (1) having a low reflectance and satisfying the condition (R1b / R1a)>； (R2b / R2a) is provided. Here, R1a is the reflectance of the background tissue with respect to the illumination light in the first wavelength band, R1b is the reflectance of blood with respect to the illumination light in the first wavelength band, and R2a is relative to the illumination light in the second wavelength band The reflectance of the background tissue, R2b, is the reflectance of blood with respect to the illumination light in the second wavelength band.小腸における微細な出血も感度よく抽出して、不明出血の患者の出血の有無を容易に発見する。波長帯域の異なる２種類の照明光を発生する照明部（２）と、該照明部（２）からの２種類の照明光の波長帯域にそれぞれ感度を有する撮像部（３）とを備え、照明部（２）からの第１の波長帯域の照明光に対して、血液および背景組織が所定の閾値より高い反射率を有し、第２の波長帯域の照明光に対して、血液が閾値より低い反射率を有し、条件（Ｒ１ｂ／Ｒ１ａ）＞（Ｒ２ｂ／Ｒ２ａ）を満足する生体内観察装置（１）を提供する。ここで、Ｒ１ａは、第１の波長帯域の照明光に対する背景組織の反射率、Ｒ１ｂは、第１の波長帯域の照明光に対する血液の反射率、Ｒ２ａは、第２の波長帯域の照明光に対する背景組織の反射率、Ｒ２ｂは、第２の波長帯域の照明光に対する血液の反射率である。
