A dieting support system comprising: a measurement function that comprises a selective semiconductor sensor to detect a component of biogas, the measurement function configured to measure concentration of the component of biogas by the selective semiconductor sensor; an analyzing function configured to: accumulate a measured result by the measurement function, and compare the measured result with a predetermined threshold value, wherein the predetermined threshold value is a value calculated in advance, or the predetermined threshold value is derived from the accumulated measured results made in the past; a timing determination function that determines suitable timing for taking a meal or for doing exercise, depending on an analyzed result by the analyzing function and other, previously accumulated data; and at least one of a meal-intake determination function that determines whether the meal is taken within a predetermined time period, or an exercise execution determination function that determines whether the exercise is done within a predetermined time period, depending on the analyzed result by the analyzing function, wherein, in response to detecting, by the analysing function using the selective semiconductor sensor, a rise of concentration of one or more gas components among ethanol, methyl mercaptan, and hydrogen sulphide, which are discharged from a living body, the meal-intake determination function determines whether the meal is taken within the predetermined time period, and wherein, in response to detecting, by the analysing function using the selective semiconductor sensor, a rapid increase in the concentration of the acetone or concentration of isoprene, wherein the acetone or the isoprene is discharged from a living body within the predetermined time period, the exercise execution determination function determines whether the exercise is done within the predetermined time period. a measurement function that comprises a selective semiconductor sensor to detect a component of biogas, the measurement function configured to measure concentration of the component of biogas by the selective semiconductor sensor; an analyzing function configured to: accumulate a measured result by the measurement function, and compare the measured result with a predetermined threshold value, wherein the predetermined threshold value is a value calculated in advance, or the predetermined threshold value is derived from the accumulated measured results made in the past; accumulate a measured result by the measurement function, and compare the measured result with a predetermined threshold value, wherein the predetermined threshold value is a value calculated in advance, or the predetermined threshold value is derived from the accumulated measured results made in the past; a timing determination function that determines suitable timing for taking a meal or for doing exercise, depending on an analyzed result by the analyzing function and other, previously accumulated data; and at least one of a meal-intake determination function that determines whether the meal is taken within a predetermined time period, or an exercise execution determination function that determines whether the exercise is done within a predetermined time period, depending on the analyzed result by the analyzing function, wherein, in response to detecting, by the analysing function using the selective semiconductor sensor, a rise of concentration of one or more gas components among ethanol, methyl mercaptan, and hydrogen sulphide, which are discharged from a living body, the meal-intake determination function determines whether the meal is taken within the predetermined time period, and wherein, in response to detecting, by the analysing function using the selective semiconductor sensor, a rapid increase in the concentration of the acetone or concentration of isoprene, wherein the acetone or the isoprene is discharged from a living body within the predetermined time period, the exercise execution determination function determines whether the exercise is done within the predetermined time period.