Recently, we developed a stable mode-locked laser oscillator with sub-100 fs timing jitter and are developing a low timing jitter regenerative amplifier system. In the development of the low timing jitter optical systems, it is necessary to establish the jitter measuring method with high accuracy. The timing jitter can be measured with high accuracy from the higher-order correlation functions. It however takes a lot of time to obtain such higher-order functions in the optical pulse train. One solution for this problem is a comparison between the amplified pulse timing and the well controlled oscillator's pulse timing; thus the timing jitter evaluation by means of the optical cross-correlation by single-shot correlation method settles such problem. However, the oscillator pulse as a timing reference is too weak to apply the conventional single-shot cross-correlation methods. We look at an example of upconversion light generation from two optical pulses. One pulse is a transform limited pulse, and the other pulse is a linear-chirped pulse. The center wavelength of the generated upconversion light depends on a relative time lag between these fundamental pulses. Therefore, if one side pulse repeats in perfect timing, the arrival time and its fluctuations of the other side pulse can be obtained by measuring a spectrum of upconverted light. Hence the arrival timing of each amplified pulse (signal pulse) can be measured by cross-correlation with the well controlled oscillator pulse (gate pulse).