PURPOSE: To obtain a dust amount measuring instrument which can indicate the dust content of air in real time and, at the same time, which can also accurately indicate the increasing or decreasing trend of the dust content.

CONSTITUTION: A collimator 2 is provided at the output end of a laser oscillating section 1 and, at the same time, an optical sensor 3 is arranged in the emitting direction of laser light through a space. An amplifier 4 which amplifies and converts the output signal of the sensor 4 into a voltage is provided and an intensity displaying section 5 which displays the intensity of the signal and time sequentially recording section 6 which time-sequentially records the intensity of the signal are connected to the poststage of the amplifier 4.