To provide a mutual length variable photochemical reaction device, capable of varying the light interaction length (an interaction time).

The mutual length variable photochemical reaction device is provided with a hollow reactor 1, having an aperture for incidence of light, a light-reflecting means 2 arranged inside the reactor for introducing a substance reacting with the light mutually thereto, for reflecting the incident light, and an interaction length varying means for varying the interaction length between the light and the substance for generating a necessary and sufficient change in the substance. In this way, the interactive length between the substance inside the reactor and the light incident on the inside the reactor can be controlled, and trace components can be measured accurately and safely, while the photochemical reaction inside the reactor can be controlled with accuracy.