A spectrophotometer 1 comprises a control unit 45 with a warming-up determination unit 452 that determines the completion of a warming-up based on a variation amount of a detection signal in a predetermined duration when a light detector 7 detects a light from a sample chamber without loading the sample. Specifically, the warming-up determination unit 452 calculates a difference between a signal intensity of the detection signal detected by the light detector 7 at the time when the predetermined time passes and a signal intensity of the detection signal detected by the light detector 7 at a previous time and determines that a warming-up is complete when a value of the difference is less than a first threshold value. The warming-up determination unit 452 automatically determines the completion of warming-up independently from the determination by the user.