In approaching the value of a real estate investment through the income, the most important matter is to credibly determine the market rent level. The current practice of taking over from different public data sources, a level of the market rent from an interval of contractual or requested rents and using it as a representation of a market rent is not appropriate, out of at least two reasons, respectively: (a) the real estate investment subject of the assessment has some features that can be substantially different from the features of comparable real estate investments; and (b) in the current practice, the real estate investors grant many facilities in order to attract and maintain the tenants, which makes the effective rent to be different from the displayed rent.The authors try to describe and argue the need of taking into account these two reasons.