Car-sharing system has two forms that are as follows: (i) Using one car by turns (ii) Using one car at the same time The former is suitable to improve the rate of operation of cars while the latter is effective to increase the rate of passengers who get on a car. Most of currently available car-sharing services belong to the type (i). This type is not fit to relief the traffic congestion in specific time (for example, at a peak period in morning). Type (ii) is more important to reduce the number of traffic volume at the same peak period. The type (ii) was adopted in this analysis. It was matter to know whom a passenger should share the car with to arrive at his/her destination by the usual time. To estimate the probability to share a car, time-space prism was applied in this analysis. When the arrival time and the destination of a passenger is included in first passengers prism, it means sharing the car by these passengers is possible. This type of car-sharing makes it possible for more people to move by the same number of cars. To increase the efficiency of the traffic, the first passenger who should drive has to depart earlier than usual. We made it clear the relationship between the driver's loss of time and the change of the efficiency.