A body fluid constituents measurement device for optically measuring a quantity of a predetermined constituent in a body fluid specimen by means of colorimetry, comprising: a mounting unit configured to be mounted with a test paper holding a coloring reagent which is reactive to the predetermined constituent of body fluid, a light-emitting element configured to emit irradiating light to said test paper, a light-receiving element configured to receive reflected light from said test paper, a determination unit configured to set light emission conditions for driving said light-emitting element, a driving control unit configured to control driving of said light-emitting element according to the light emission conditions set by said determination unit, and a temperature measurement unit configured to measure ambient temperature in the vicinity of said light-emitting element, wherein said body fluid constituents measurement device is configured to perform measurement of the quantity of the predetermined constituent in the body fluid specimen by detecting, at said light-receiving element, a reflected light intensity from said test paper where color change occurs according to the quantity of the predetermined constituent of the body fluid, characterized in that said determination unit is configured to set a first light emission condition for driving said light-emitting element prior to performing the measurement during a predetermined period of time after the device is switched on and before the measurement is started, based on the ambient temperature measured by said temperature measurement unit when said body fluid constituents measurement device is switched on wherein said first light emission condition is different from a second light emission condition for driving said light-emitting element during performing the measurement, after said predetermined period of time, and said driving control unit is configured to control driving of said light-emitting element according to said first light emission condition for said predetermined period of time for light intensity stabilization processing prior to the measurement, and thereafter to control driving of said light-emitting element according to said second light emission condition for measurement processing during the measurement. a mounting unit configured to be mounted with a test paper holding a coloring reagent which is reactive to the predetermined constituent of body fluid, a light-emitting element configured to emit irradiating light to said test paper, a light-receiving element configured to receive reflected light from said test paper, a determination unit configured to set light emission conditions for driving said light-emitting element, a driving control unit configured to control driving of said light-emitting element according to the light emission conditions set by said determination unit, and a temperature measurement unit configured to measure ambient temperature in the vicinity of said light-emitting element, wherein said body fluid constituents measurement device is configured to perform measurement of the quantity of the predetermined constituent in the body fluid specimen by detecting, at said light-receiving element, a reflected light intensity from said test paper where color change occurs according to the quantity of the predetermined constituent of the body fluid,