To provide a compost agitator capable of sufficiently agitating a large quantity of compost with a small-sized agitator, efficiently supplying air necessary for the fermentation of the compost into the compost and suppressing the spread of malodor produced from the fermented compost to the air.

A high pressure gas is diffused in the compost by providing a closing tool for closing the whole or a part of a vertical hole formed in the vertical direction in the compost by the rotation of an agitating blade on the outer periphery of the axial directional desired position of a rotary shaft and the back surface side of the agitating blade to prevent the high pressure gas from releasing upward through the vertical hole.