PURPOSE: To accurately and easily decide the state of liquid by partitioning a liquid detection container into an upper and a lower layers and comparing the quantity of transmitted light of upper layer liquid with that of lower layer liquid.

CONSTITUTION: An aperture 3 is formed at the bottom part 2 of an oil pan 1 and the detection container 4 is fitted. The container 4 is partitioned by a partition plate 5 into an upper chamber 6 and a lower chamber 7 and plural small holes 5a are pierced in the partition plate 5. Oil 9 deteriorates in state with time owing to the mixing of foreign matter such as blackened dirt and metal powder and the deteriorating upper layer liquid 6a is collected in the upper chamber 6. The small holes 5a are small, so lower layer liquid 7a which do not deteriorate with time is collected in the lower chamber 7. Then, projectors 10 and 12 are provided to the upper chamber 6 and lower chamber 7 to project measurement light, and light transmitted through the oil 9 is received by photodetectors 11 and 13. The transmissivity of the upper layer liquid 6a decreases relatively to the transmissivity of the lower liquid 7a, so the comparing circuit of a detecting circuit 18 compares the quantities of transmitted light with each other to decide the dirt state of the oil.