The space charge effect is important in free-electron devices and sometimes plays a key role. A vacuum-compatible micromanipulation platform was devised to study this effect in a diode structure while changing the width of the gap between the emitter and the collector in situ in the range of hundreds of micrometers to tens of micrometers. The current–voltage characteristics were found to upshift with a decrease in the interelectrode distance; the space charge-limited current increased by approximately two orders of magnitude when the interelectrode distance decreased from 550 to 50 μm. The simplicity of the parallel-plate structure enabled analysis based on one-dimensional emission and transport with the measurement results showing a good fit to the model by Longo combined with the Child–Langmuir theory.