You allow negative inventory: IC Setup, Options, Processing: Allow Negative Inventory Levels. Turn it off and correct your inventory costing.

Thank you so much...i just did as you sad..on the Processing page, the "Allow Negative Inventory Level" is actually not checked....I guess i made a mistake and I have rectified it...On the stock clearing area,my store man actually caused the mistake,previous month he stated that stock level is zero,and that cleared the cost, the following month,nothing was bought and he still said the quantity is one...So I had to re-enter it but i forgot to input the cost...when it was issued, i had to input the cost adjustment which caused a negative cost(the new cost 490,was subtracted from the old cost,zero)....i just perform a cost increase now...and it was ok....Thank you so much for your idea...I appreciate