[en] The next-generation synchrotron radiation source project to provide a highly brilliant soft X-ray beam for users is underway. The injector linac to inject to the low-emittance storage ring generates a high-quality beam, which is highly stable and repeatable. To produce the beam in the linac, a new low emittance thermionic electron gun system with a 238-MHz RF cavity demanded with a 42 kW RF input power has been developed. We manufactured a 42 kW semiconductor amplifier that is the RF power source for the 238-MHz RF cavity. This amplifier comprises an ultra-low noise power supply, an RF part and a control part. The power supply greatly helps the highly stable RF output of the amplifier. The RF part is composed of semiconductor amplification modules having the several stages of laterally diffused metal oxide semiconductor (LDMOS) field-effect transistors (FET). In a test, an RF power of 42 kW with a pulse width of 100 μs was outputted through a reentrant cavity type RF combiner, which has one output and 36 input ports connected to the output of the amplifier modules. In this report, we describe the details of the design and the test result for the RF amplifier. (author)