An embodiment of the present invention provides a method for providing a real estate transaction service to each of a lessor and a lessee by a rental manager server, the method comprising the steps of : (a) receiving, from a lessor terminal, a lessor' s lease contract request including a first security deposit amount desired by the lessor, a first monthly rental fee amount desired by the lessor, and information relating to real estate, which the lessor wants to lease, and then determining a reference rental price of the real estate, which the lessor wants to lease; (b) confirming a first initial security deposit amount and a first initial monthly rental fee amount within a range equivalent to the reference rental price by referring to the first security deposit amount desired by the lessor and the first monthly rental fee amount desired by the lessor, and concluding a lessor' s lease contract including, as a contract condition, the confirmed first initial security deposit amount and first initial monthly rental fee amount; (c) receiving, from a lessee terminal, a lessee' s lease contract request including a second security deposit amount desired by the lessee, a second monthly rental fee amount desired by the lessee, and information relating to real estate, which the lessee wants to rent; and (d) confirming a second initial security deposit amount and a second initial monthly rental fee amount within the range equivalent to the reference rental price by referring to the second security deposit amount desired by the lessee and the second monthly rental fee amount desired by the lessee, and concluding a lessee' s lease contract including, as a contract condition, the confirmed second initial security deposit amount and second initial monthly rental fee amount.