I suspect that your data in the cells in the worksheet column have been stored as text strings rather than the numeric values. In a typical situation, the former appear left-adjusted, and later appear right-adjusted in the cells. You can double-click the column header of your data to show the Column Properties dialog, and change the column's "Format" to "Numeric" rather than the default "Text & Numeric" to force them to be converted to the numbers.
I hope my suspect is your actual situation. If it isn't, please describe where is your data, and in which way you have tried to convert your data.
Thank you Hideo! I resolved this problem before in same way as you proposed. I was just wondered what's wrong. I thought that if I try to use function which is not defined I get error text in the Command Window.