We developed a compact, high-peak-power picosecond pulse source based on a gain-switched laser diode with an emission wavelength of 980 nm and successfully demonstrated two-photon fluorescence imaging. The pulse source consisted of the gain-switched laser diode, a pulse compressor, a pulse reshaper, and optical amplifiers, all connected by single-mode fibers. The optical pulse generated by the pulse source had a pulse width of 3.5 ps, an average power of 35 mW, and kilowatt-level peak power at a repetition rate of 10 MHz. Alexa488-stained Glomeruli and convoluted tubules of a mouse kidney section were observed by two-photon imaging with the developed pulse source.