To provide a simple slurry-density adjusting device capable of easily adjusting density rapidly.

The slurry-density adjusting device has the first slurry tank 1, the second slurry tank 2 in which a fluid is communicated movably with the first slurry tank 1, a water-level elevating means 3 elevating a water level in the second slurry tank 2 by sucking a gas or the liquid in the second slurry tank 2, a soil-sand supply means 4 supplying the inside of the first slurry tank 1 with soil and sand and a screen means 7 passing only slurry containing soil and sand having fixed grain size or less existing in the first slurry tank 1 through the second slurry tank 2.