▎ 摘　　要

Utilizing a complete Lorentz-covariant and local-gauge-invariant formulation, we discuss the response of graphene to arbitrary external electric fields. The relation, which we call here the Kramers-Kronig relation, between the imaginary part and the real part of ac conductivity is given. We point out that there exists an ambiguity in the conductivity computing, attributed to the 'poor' behavior in the ultraviolet region. We argue that to study the electrical response of graphene completely, the non-perturbational contribution should be considered.