As is well known, free electron lasers (FELs) can generate coherent light at any wavelengths in principle, and its spectral range covers from terahertz radiation to x rays. As in traditional optical lasers, reducing the pulse length is an important subject in the development of FELs, and generation of attosecond pulses has become possible in LCLS, one of the X-ray FEL facilities currently in operation. It should be noted, however, that reducing the FEL pulse length down to a few-cycle duration is a great challenge; besides technical issues, there exists a theoretical limit that disturbs the realization of few-cycle FELs. This is known as a "slippage" effect, in which radiation overtakes electrons while they move in a periodic magnetic field to amplify radiation. Recently, we proposed an idea to overcome this difficulty and experimentally demonstrated it. In this talk, I will review its fundamental mechanism and report the results of the demonstration experiments, together with perspectives of few-cycle attosecond pulses that will be available with this concept.