Time evolution of a white-light flare accompanied by probable postflare loops on the M-type dwarf EV Lacertae

White-light flares are explosive phenomena accompanied by brightening of the continuum from near-ultraviolet to optical, which occur on the Sun and stars. In order to investigate the mechanism of white-light flares, we carried out simultaneous optical photometry (TESS: 6000–10000 Å) and spectroscopy (Seimei telescope: 4100–8900 Å) of the active flaring M-dwarf EV Lacertae on 2019 September 14. We detected a flare with high-time-cadence ($\sim$50 s) spectroscopic observation. At the peak, the continuum of the flare component is well fitted by a blackbody spectrum with a temperature of $T = 8122 \pm 273$ K, which is comparable with the results of previous studies, which reported that the spectral energy distribution of near-ultraviolet to optical during the flare could be approximated by single-temperature blackbody radiation at $T \sim 10^{4}$ K. We also estimated the time evolution of the flare temperature during the decay phase, which has not been reported with high-time-cadence spectroscopy. The radiative energy of this flare within the optical range is $4.4 \times 10^{32}$ erg, taking into account the time-dependent variation in the decreasing flare temperature and expanding flare area. Furthermore, we detected a delayed increase in the flux of H$\alpha$ after the photometric flare peak, secondary increase, and gradual increase even after the white-light flare ended. Comparison of our results with light curves obtained by the Sun-as-a-star analysis of solar flares indicates that these signals may be due to postflare loops near the stellar limb. Our result for the time evolution of the white-light continuum will help us to gain more insight into the mechanism of white-light flares on both the Sun and stars. Additionally, since extreme ultraviolet radiation from flare loops plays a key role in planetary atmospheric escape, the existence of postflare loops on stellar flares and their time evolution will help future studies on the habitability of close-in planets.