mileage reimburstment

Hi guys, i have a question, hope some of you can help.
On my job, there are times I need to drive to different locations to get somethings or return some; or I need to drive to different city for preparing opening new locations.
I used to get reimburstment for mileage from my former supervisor, but not from the current one due to the excuse "no budget".
My question is, if my employer is obligated to reimburst such expenses, which are related to job. My job function doesn't require frequent driving, especially on personally expense.
thank you all in advance