Casting investments are used by mixing a powder and liquid, and there is a risk of fine powder inhalation causing respiratory disease. Previously, we investigated paste-paste type phosphate-bonded investment with oils. The objective of the present study was to produce paste-paste type phosphate-bonded investments with colloidal silica. Two pastes (PA and PB) were formed before the test. PA was an acidic mixture of cristobalite and magnesium dihydrogenphosphate solution, and PB was an alkaline mixture of MgO in colloidal silica solution. Five experimental investments (PB5, PB10, PB15, PB20, PB25) containing 5, 10, 15, 20, and 25 ml, respectively, of colloidal silica in PB were evaluated. Commercial phosphate-bonded investment was used as a control. PB5 took a long time to set (12 h or more), had no setting expansion, and its compressive strength was very small. Its thermal expansion and XRD analysis results were almost the same as those from the other experimental investments. The setting expansions of PB20 and PB25 were 0.40% and 0.62%, respectively, and they took approximately 16 min to set. These values were close to those of control. Full coverage cast crowns obtained from the experimental investments had a loose fit compared with the control. The gap with the crowns cast from PB20 was smaller than those of other experimental investments, and it showed no significant difference compared to that of the control. These results suggest that it is possible to produce pastepaste type investments using colloidal silica. This limited study suggests that the experimental investment with PB20 is suitable for clinical use.