A capsule body-insertable apparatus (10, 10A, 10B, 10C, 10D) to be inserted into a subject, the capsule body-insertable apparatus comprising: a battery and a power supply system installed in the capsule body-insertable apparatus; an illumination unit (13, 13a, 13b) configured to illuminate an inside of the subject; an imaging device having a light shielding board that defines an effective pixel region (R1, R11, R eff) on which an optical image of the inside of the subject illuminated by the illumination unit (13, 13a, 13b) is to be formed and an optical black region (R3, R13, R OB) at which the optical image is to be shielded; and a light adjustment control unit (16, 17, 18) including: a measured light value calculating unit configured to calculate a measured light value from the effective pixel region (R1, R11, R eff); a target value storage unit (17, 17a, 17b) configured to store a target value of a drive amount for driving the illumination unit (13, 13a, 13b); a black level detecting unit (18, 18a, 18b) configured to acquire a black level index of a black level of the optical black region based on pixel values, each pixel value being the simple average, the weighted average, or the median of a specific region of each line contained in the optical black region (R3, R13, R OB); and a light adjustment computing unit (16, 16B, 16a, 16b) configured to adjust the amount of light from the illumination unit (13, 13a, 13b) based on the measured light value calculated by the measured light value calculating unit, the target value stored in the target value storage unit (17, 17a, 17b), and the black level index acquired by the black level detecting unit (18, 18B, 18a, 18b). a battery and a power supply system installed in the capsule body-insertable apparatus; an illumination unit (13, 13a, 13b) configured to illuminate an inside of the subject; an imaging device having a light shielding board that defines an effective pixel region (R1, R11, R eff) on which an optical image of the inside of the subject illuminated by the illumination unit (13, 13a, 13b) is to be formed and an optical black region (R3, R13, R OB) at which the optical image is to be shielded; and a light adjustment control unit (16, 17, 18) including: a measured light value calculating unit configured to calculate a measured light value from the effective pixel region (R1, R11, R eff); a target value storage unit (17, 17a, 17b) configured to store a target value of a drive amount for driving the illumination unit (13, 13a, 13b); a black level detecting unit (18, 18a, 18b) configured to acquire a black level index of a black level of the optical black region based on pixel values, each pixel value being the simple average, the weighted average, or the median of a specific region of each line contained in the optical black region (R3, R13, R OB); and a light adjustment computing unit (16, 16B, 16a, 16b) configured to adjust the amount of light from the illumination unit (13, 13a, 13b) based on the measured light value calculated by the measured light value calculating unit, the target value stored in the target value storage unit (17, 17a, 17b), and the black level index acquired by the black level detecting unit (18, 18B, 18a, 18b). a measured light value calculating unit configured to calculate a measured light value from the effective pixel region (R1, R11, R eff); a target value storage unit (17, 17a, 17b) configured to store a target value of a drive amount for driving the illumination unit (13, 13a, 13b); a black level detecting unit (18, 18a, 18b) configured to acquire a black level index of a black level of the optical black region based on pixel values, each pixel value being the simple average, the weighted average, or the median of a specific region of each line contained in the optical black region (R3, R13, R OB); and a light adjustment computing unit (16, 16B, 16a, 16b) configured to adjust the amount of light from the illumination unit (13, 13a, 13b) based on the measured light value calculated by the measured light value calculating unit, the target value stored in the target value storage unit (17, 17a, 17b), and the black level index acquired by the black level detecting unit (18, 18B, 18a, 18b).