Indesit K3G21XG oven does not stay on?

indesit oven does not stay on when i use the oven/grill botton on the cooker .Button which works both the oven & grill' if i press and hold the oven & grill button and turn it to the right the the grill works OK. But if i turn it to the left an hold it in and try to light the oven it lights the gas for as long as i hold it in . But as soon as i let go of the button the oven goes out . But if i hold the button it in again it lights the gas then goes out when i let go.