In an engine generator unit, a framework (11) has vertical pipe frame portions (12a, 13a) at its four corners, and an engine (40) and an electric-power generator (50) to be driven by the engine (40) are provided coaxially in a direction of an engine output shaft (41) within a space defined by the framework (11). Fuel tank (90) is provided above the engine (40) and electric-power generator (50) between a pair of support members (127, 128) each spanning between a different pair of the vertical pipe frame portions (12a, 13a). Muffler (102) connected to an exhaust-discharging end of the engine (40) is positioned adjacent the fuel tank (90) in a side-by-side relation to the fuel tank (90). Heat blocking cover (121) covers top and side portions of the muffler (102) and is placed between the pair of support members (127, 128) in such a way that a substantially entire top region of the space defined by the framework (11) is covered with the fuel tank (90) and the heat blocking cover (121). Thus, it is possible to increase the capacity of the muffler (102) while providing for a large capacity of the fuel tank (90).