To provide a soil improving material feeder capable of suppressing a lowering of accuracy in feeding a soil improving material when filling the soil improving material.

The soil improving material feeder 14 used for a soil improving machine for improving sediment by mixing the soil improving material includes a storage tank 15 storing the soil improving material; a chute 17 continuously provided at the lower part of the storage tank 15 and reduced in diameter toward a lower part; and a screw feeder 16 feeding the soil improving material in the storage tank 15 guided by the chute 17, to the sediment. The screw feeder 16 includes a casing 61; a screw 62 rotatably provided in the casing 61; a driving device 63 rotationally driving the screw 62 connected to a rotating shaft 66 of the screw 62; and a shielding member 75 provided at the peripheral edge part of the screw 62 and facing a clearance between the facing parts of the inner peripheral surface of the casing 61 and the outer peripheral part of the screw 62 to reduce the clearance.