A method of estimating strength of forming photochemical ozone utilizing a pump and probe technique is disclosed along with an apparatus for estimating strength of forming photochemical ozone utilizing a pump and probe means whereby strength of forming photochemical ozone can be estimated without measuring a concentration of each type of various reactive hydrocarbons in order to predict strength of forming photochemical ozone from rates of their reaction with OH radicals. Atmosphere (11) is irradiated with a pumping laser light (2c) to photolyze ozone therein, thereby forming excited oxygen atoms O(.sup.1D) which are then caused to react with water vapor to form OH radicals. The OH radicals are irradiated with a probing laser light (3e) to excite electrons therein. A fluorescent light emitted when the excited electrons are relaxed to transition is measured in a time sequence. The intensity of fluorescence being proportional to the concentration of the OH radicals, the strength of forming photochemical ozone can be estimated from the change of the concentration of the OH radicals with time.