In a microwave multistage amplifier, a shield casing (8) has a partition plate (7) integrally moulded with and disposed in the casing (8); microwave amplifiers (5, 6) are arranged on a dielectric substrate so as to locate the partition plate therebetween; and an upper cover plate (11) is provided which can be screwed to partition plate (7). By virtue of the above-mentioned arrangement, it becomes possible to strictly separate rooms defined in the casing from each other and suppress generation of a surface wave mode of the microwave propagation on the dielectric substrate, thereby assuring an isolation between the amplifiers.