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Nanosecond and sub-nanosecond high voltage pulses can provide new applications. A cancer treatment by an ultra-short pulse high electric field is one of them. High power pulsed microwave has been proposed to apply the high electric field for that treatment. This work focuses on the design of a compact high power pulsed microwave generator using a nanosecond pulse power generator for the cancer treatment. To obtain fast rise time of voltage and current for nanosecond pulses, a switch has to have low inductance. Water gap switches have this property. Since water has a dielectric strength exceeding 1 MV/cm, gap distances for switching can be reduced to several hundreds of micrometers, and still allow switching of tens of kV. The narrow gaps allow us to reduce the switch inductance. In this study, the water gap switch was built in a Blumlein pulse forming line. The Blumlein line was designed to provide a pulse of 1 ns and to match an impedance to 16 Omega of an antenna. By using the water gap switch in the Blumlein line, the voltage rise time obtained approximately 750 ps at 13 kV of a peak pulse voltage. An electromagnetic wave was radiated underwater by the loop antenna. A measuring antenna at 0.15 m and 0.4 m from the radiating antenna caught electric field intensities of 116 kV/m and 32 kV/m, respectively. A frequency element of 250 MHz had higher intensity.