An electron-capture supernova (ECSN) is a core-collapse supernova (CCSN) explosion of a super-asymptotic giant branch (SAGB) star with a main-sequence mass $M_{rm ms}sim7-9.5M_odot$. The explosion takes place in accordance with core bounce and subsequent neutrino heating and is a unique example successfully produced by first-principle simulations. This allows us to derive a first self-consistent multicolor light curves of a CCSN. Adopting the explosion properties derived by the first-principle simulation, i.e., the low explosion energy of $1.5times10^{50}$ erg and the small $^{56}$Ni mass of $2.5times10^{-3}M_odot$, we perform a multigroup radiation hydrodynamics calculation of ECSNe and present multicolor light curves of ECSNe of SAGB stars with various envelope mass and hydrogen abundance. We demonstrate that a shock breakout has peak luminosity of $Lsim2times10^{44}$ erg/s and can evaporate circumstellar dust up to $Rsim10^{17}$ cm for a case of carbon dust, that plateau luminosity and plateau duration of ECSNe are $Lsim10^{42}$ erg/s and $tsim60-100$ days, respectively, and that a plateau is followed by a tail with a luminosity drop by $sim4$ mag. The ECSN shows a bright and short plateau that is as bright as typical Type II plateau supernovae, and a faint tail that might be influenced by spin-down luminosity of a newborn pulsar. Furthermore, the theoretical models are compared with ECSN candidates: SN 1054 and SN 2008S. We find that SN 1054 shares the characteristics of the ECSNe. For SN 2008S, we find that its faint plateau requires an ECSN model with a significantly low explosion energy of $Esim10^{48}$ erg.
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