Generally, the ultimate strength of restrained concrete square slabs express larger value than the bending strength obtained from the yield line theory of Johansen. This cause is explained by an arching action due to the development of compression membrane force in the slab. Therefore, in this report we described the relation between the evaluation value and its effecting factors: The evaluation value (E) express a strength grow due to the development of compressive membrane force in the slab. The effecting factors were considered by the items of three: Slab's thickness/span ratio (H/L) or flange thickness/span ((H-h)/L), elastic fixing degree (F_<ix>) and reinforce factor (q). We investigated the relation between the evaluation value and effecting factors as follow: 1) Relation between evaluation value (E) and slab's thickness/span or flange thickness/span ((H-h/L). 2) Relation between evaluation value (E) and elastic fixing degree (F_<ix>). 3) Relation between elastic fixing degree (F_<ix>) and slab's thickness/span ratio (H/L) or flange thickness/span ratio ((H-h)/L). 4) Relation between evaluation value (E) and reinforce factor (q). The results was shown as follow: 1) As the value of slab's thickness/span ratio (H/L) or flange thickness/span ratio ((H-h)/L) increases, the evaluation value (E) showed an increasing tendency. Therefore, the evaluation value (E) was very closely related with slab's thickness/span ratio (H/L) or flange thickness/span ratio ((H-h)/L). 2) As the value of reinforce factor (q) increases, the evaluation value (E) showed a decreasing tendency. 3) Elastic fixing degree (F_<ix>) was very closely related with slab's thickness/span ratio (H/L) or flange thickness/span ratio ((H-h)/L). 4) Elastic fixing degree (F_<ix>) gave an effect in the strength of slab by restrained degree of rotation with surround restrained beam. But it was found that the elastic fixing degree (F_<ix>) gave a small effect to the evaluation value (E).