In actual OFDM trasmitter, is the stepwise waveform after IFFT put on the carrier as it is?
It's hard to say whether something is wrong with your transmitter without knowing exactly what this picture depicts, but yes, the spectrum of a single subcarrier is sinc-shaped.


  the spectrum of it is not white(flat shape). Why?


First of all:

OFDM isn't a flat top spectrum – it only looks like that from "far away", or if you're sampling it at the same rate as the transmitter generated it (which requires synchronization!).

In general, you'll see the "wells" between the sinc carriers, relatively well. It looks the flatter the more subcarriers you have.

In your case:

Your picture looks like you're only looking at a single subcarrier, which means you're effectively doing 1-carrier OFDM. Which is nothing but a box-car pulse-shaped single-carrier transmission. Hence, the sinc shape.