This invention provides a monitor device capable of detecting possible abnormalities in body fluid withdrawal by a body fluid sampler and variations in the dilution when a withdrawn body fluid is diluted, and a living organism-measuring device with this monitor device. The former device includes a first optical sensor disposed on a diluent channel through which a diluent is introduced into a body fluid sampler for withdrawing a body fluid; a second optical sensor disposed on a diluted body fluid channel through which the diluted body fluid is transferred to a living organism-measuring sensor, and close to the first optical sensor; and a controlling unit for judging whether variations in a dilution to which the body fluid is diluted are within predetermined limits, from data outputted by the first optical sensor and data outputted by the second optical sensor. The latter device has the monitor device incorporated thereto.