Can't acess 64bit dll in VSTO COM addin

I want to access mydll.64 in a visual studio 2017 VSTO project but its not working and displaying some error like..

(An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)


************** Exception Text **************
System.BadImageFormatException: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)
   at ExcelAddIn1.clsDLLBasedConv.CUGetTotalPQs()
   at ExcelAddIn1.Form1.LoadPhysicalQuantities() 

Please help me to solve this issue.