We observed two-dimensional structure and time evolution of ionospheric irregularities caused by midlatitude sporadic E (Es) over Japan as positive anomalies of total electron content (TEC) by analyzing the data from the nationwide Global Positioning System (GPS) array. In this paper we report a case study of strong Es observed in the local evening of 21 May 2010, over Tokyo, Japan. In the slant TEC time series, Es showed a characteristic pulse-like enhancement of ~1.5 TEC units lasting for ~10 min. We plotted these positive TEC anomalies on the subionospheric points of station-satellite pairs to study the horizontal structure of the Es irregularity. We confirmed that the irregularity existed at the height of ~106 km by comparing the data of multiple GPS satellites, which is consistent with the local ionosonde observations. The horizontal shapes of the Es irregularity showed frontal structures elongated in E-W, spanning ~150 km in length and ~30 km in width, composed of small patches. The frontal structure appears to consist of at least two parts propagating in different directions: one moved eastward by ~60 m s−1, and the other moved southwestward by ~80 m s−1. Similar TEC signatures of Es were detected by other GPS satellites, except one satellite that had line of sight in the N-S direction which dips by 40–50° toward north, which indicates the direction of plasma transportation responsible for the Es formation. We also present a few additional observation results of strong Es irregularities.