When sampling light and light to be measured each having a wavelength longer than an upper wavelength limit of the sensitivity of a photocathode are incident on the photocathode of a multiphoton absorption-type electron tube, light-light sampling is enabled by two-photon absorption and one-photoelectron emission. A delaying unit adjusts an incident timing of the sampling light from an optical pulse generating unit on the elctron tube in synchronism with an incident timing of the light to be measured on the electron tube. A half mirror causes the sampling light and the light to be measured to coincide on the photocathode of the electron tube. A controlling unit controls the delaying unit to sequentially shift the incident timing of the sampling light on the electron tube. Thus, the operation of a display unit can be controlled in synchronism with the incident timing of the sampling light so that a series of sampling outputs from the electron tube can be converted to the optical waveform of the light to be measured and displayed on the display unit.